I haven't scaled it myself, but I've read that it's closer to 14 meters long in F mode and 10 meters tall in B. An average Zentraedi is around 9.5 meters tall. Breetai and Golg Bodolza are much taller, like the Zentraedi equivalent of basketball players.
I'm not bothering with scaling. This is direct from supplementary materials. (They also give average Zentradi as between 40 and 50 feet, and Bodolza as nearly ninty.)
Which supplementary materials? Were they from Macross or Robotech (Robotech grossly overstated the size of Zentraedi numerous times)? IIRC, Global described the Zentraedi as "five times the height of a human", which would be around 9 meters. The site
www.new-un-spacy.com corroborates this.
Even at their smallest, they're 250lb bomb size, that's still bigger than most vehicle-fired ATGMs like TOW. These are big weapons, complex, hard to produce. We actually do see Regults survive direct hits from these weapons on rare occasions and retain enough functionality to limp back to base, so a mission-kill with a smaller one will not be simple.
What about land mines or IEDs? You can build a much bigger bomb if all you have to do is set it down and detonate it remotely. It's certainly more accessible to a struggling insurgent force with no remaining industrial base than a
transforming airplane.
That's true, but they did manage to repair crash damage when Hikaru was being a total klutz at the controls in the first few episodes, and it took a good bit of actual combat damage to stop his VT-1D. They were also designed to engage in hand-to-hand combat if needed. They have a level of shock-damage resistance that means they're basically durable machines and hard to break, more like tanks than aircraft. Since we don't know much about overtechnology, I think you're making a few assumptions that may not be warrented.
I don't see how it could possibly be unwarranted that a non-transforming fighter would have major performance advantages. Fewer moving parts (thus fewer parts that can fail), less mass devoted to articulation machinery and more mass that can be allocated to armor without making the machine unreasonably heavy, more internal space to allocate to armor and components, etc. These are
extremely fundamental engineering principles that will not be invalidated by any new technologies, ever.
Which was why they planned what they did, but it would take a ridiculously huge number, tens of millions, just to deal with Asia. Moving that number of people interstellar distances is tough, moving Zentradi is exponentially tougher because of their size. They planned for a reasonable force to take a planet in a conventional campaign. The force that came was totally unreasonable.
Point conceded, but good luck even attempting to conceal an aircraft manufacturing plant from orbital or aerial surveillance. A small base might not get caught, but you aren't going to hide something like that, and you have to build VF-1s somewhere if you want to replace losses.
The Zentradi show a level of warmaking sophistication in their designs that belies this tendancy. Their equipment was advanced and relatively well-designed, which suggests they have gone beyond such a simplistic way of war. It was not the UN SPACY's to know that the Zentradi didn't actually make their own equipment. They had to make a guess based on what they knew. It was wrong, but it was not bad.
Indiscriminate destruction and spite are not necessarily features of primitive barbarians. Simply put, if the Zentraedi invade Earth and encounter a quagmire, it is a big problem for them. It ties up resources they could use to fight other enemies, it exhausts the soldiers' patience, it looks bad for them and hurts morale, and it's generally an inconvenience. If securing Earth is difficult enough, why not just kill them all and end the pointless waste of lives and materiel? They gain nothing from Earth if the pesky miclones keep making a mess of everything they try to do with it. It's not like the Zentraedi have any concept of sentient rights or anything, and to expect them to would be the height of folly--to the pre-war UN they might, but it's best not to count on it. Advanced technology is not a package deal with "modern" scruples or concern for the lives of those outside the group. There's no reason to expect them to give a damn if all the miclones die.
Yet A is the only rational object to select, as any other option leaves you in a significantly weaker posisition come the next round; and there will be a next round with any other option. Ultimately A is the goal, so you prepare for that one while having a contingency plan for B and C that can be executed if required. If given another twenty years then undoubtedly specialized designs would have appeared and composed the forces that would have opposed the Zentradi fleets. But they needed a new, interim weapon now that can do all these things, at once, while they learn how to build better and bigger and tougher and more powerful spaceships to hold the front line when the time comes. A VF-1 is unlikely to be more expensive to develop and produce than a new space fighter, atmospheric fighter, and ground mecha all at the same time, and has advantages for the contingency plans as noted.
A is absolutely not rational--it was
impossible to achieve victory; the Zentraedi are completely superior to the UN Spacy in every meaningful way, and it was obvious from the beginning. There is a
zero chance of the UN Spacy defeating the Zentraedi in all out war; it was not going to happen, and if the UN Spacy leaders had six brain cells between them, they knew it was not going to happen (what their original strategy was, we'll probably never know, because the SDF-1 derailed it in the first couple of episodes). They might be able to stall the Zentraedi for a while, but they sure as hell weren't going to beat them. The idea of being in a stronger position the next round is irrelevant, because there isn't going to be a next round in an all-out conflict because the UN will utterly crushed.
The only options that are not doomed to total failure are to wage a guerilla insurgent campaign to bloody the Zentraedi's noses and cause a quagmire (but this risks provoking annihilation), to surrender (if they accept it), or to flee and hope the Zentraedi don't follow them (ideally, the escape fleet should be launched before the Zentraedi show up, and the SDF-1 should leave with it to deny the Zentraedi their reason to be there at all, but this didn't happen).
Two things: assumptions about overtechnology not warrented by facts, as noted above.
High technology comes with a higher required base and additional secondary technologies; that's just a fact of life. To build technology A, you require technologies B and C, which require technologies D, E, F, and G, and so on. There is no reason to think this would change in the future when it is the rule for pretty much every new technology in human history.
The Zentradi are aliens. It might be entirely possible to produce spares or new VFs under their noses. US forces in Iraq and Afghanistan have problems with people building bombs and carrying around weapons to use against them under their noses and the gaps of culture and langauge are significantly smaller there than with Zentradi.
Absurd. You don't need similar cultures to recognize industrial facilities when you see them; the machinery and facilities are the same whether you're American or Chinese or 30-foot-tall space aliens (although the latter's facilities might be larger). Also, you cannot compare VFs to the sort of crude weapons insurgents manufacture (and exactly the kind of weapons I described). You can build an IED out of some basic electronics (probably stolen or smuggled from another country, options not available to the UN), an ignition source (those have existed since the Stone Age), and fertilizer. A firearm can be constructed with a simple metal press; the Palestinian Jews, before Israeli independence, built thousands of Sten submachine guns on farms with little or no advanced tooling. Crude gunpowder can be made with a basic chemistry set and urine (yes, urine). The sort of facilities, techniques, and resources that go into even a modern aircraft, let alone a VF, are orders of magnitude greater and, accordingly, more difficult to conceal. The insurgents can easily build IEDs under our noses, but they don't build MiG-29s under our noses (or at all, for that matter). The IED is the classic insurgent weapon. It's crude, can be made without advanced technologies or techniques in covert sites, extremely cheap, and reasonably powerful. A variable fighter completely fails the first three of these criteria.
Again, an interim weapon able to do many things. Quick to develop, relatively simple, minimal capablity needed to overcome a Regult or whatever their baseline was. One project, one weapon, devote more people to the things that will actually hold the line once they come onstream ten or fifteen years down the road.
The UN Spacy did not have 10-15 years. The length of the war against the Zentraedi, were it not for the SDF-1 malfunctioning and throwing both the UN and the Zentraedi for a loop, would have been measured in weeks, not years. And furthermore, they already had not one but many other mecha designs that were far more capable on the ground than a VF-1. A Spartan or Monster makes the VF-1's armament look like a Nerf gun. One can only imagine that their manned overtechnology fighters were similar (the robot fighters launched from the space carriers weren't really comparable, they seemed more like disposable swarm machines than real fighters).
Yes.
But not all at once.
One platform that does five things poorly is always overshadowed by five platforms that do one thing well. A few people tried this sort of approach in WWII--it didn't work out very well, as evidenced by the M1938 Christie tank (a flying tank that was absolutely horrendous at being either an aircraft or a tank), German U-cruisers (basically a submarine/cruiser hybrid, also a failure), zeppelin aircraft carriers (a big, slow, flammable target that carried some of the most pathetic, useless fighters ever built), various attempts at "battlecarriers" (none made it to the battlefield), etc. Compromise designs are greatly inferior to designs with a clearly defined mission profile, and the drawbacks outweigh the benefits.
Can you guys please do something more productive then arguing about a bunch of stuff that never happened.
Put bluntly, why the **** do you care? 
Because it gives some people a smug sense of superiority to quash discussions because they're too geeky or something.

See also: many Something Awful goons. "Uhuhuhuh, look at those [pick an insult: retards/faggots/neckbeards/aspies/furries] arguing over Star Wars, I'm going to feel smarmy and superior even though I spent real money to get on a ****ing internet forum."
@everyone else: Jesus Christ, if you want to see other discussions here, then post something to discuss, it's not like the mods will delete your posts for not being about the validity of variable fighters now.