Ok, people have debated about the size of HW ships both in the Homeworld community and in various modding projects that bring the Homeworld universe to Freespace 2.
First off, the units in Homeworld and Homeworld: Cataclysm aren't defined that well. There was no confirmation from Relic or Sierra regarding how long these ingame "units" are. The only things that can be inferred are from various pieces of official art for the game, some commentary from the creators, and the cutscenes.
In Homeworld 2, this is different. The game designers confirmed that the units being used are meters. The only way to estimate how these units stack up to HW1/Cata units is to go off comparison of similar units, as there are no designs shared in either game.
The HW2 Hiigaran interceptor is 17 units long, therefore is 17 meters long. The Kushan interceptor from the original Homeworld game is 58 HW1 units long. If this is meters, that's over half the length of a football field. So either the Kushan and Taiidani were flying these ships that were three times the size of their successors, or the units are different.
While one can argue that with miniaturization, the ships got smaller, for strike craft this still ends up being a huge issue. While there is no official word from Relic as mentioned, there is a ton of official art for the game that supports how large HW1 units are. There are pictures showing the open cockpit of a Taiidan scout and the pilot seat is visible, along with a similar shot of the Somtaaw Acolyte. The best one shows a Taiidan Corvette landed on a docking pad, with a side door open, and a few people standing around it. This door is clearly visible on the texture of the same corvette, and the artwork confirms it's a door and not a vent.
This door is roughly six HW1 units tall. So either Taidani and Kushans are 6 meters tall (nearly 20 feet) or the HW1 units are closer to feet. In fact, if you take the length of the HW1 interceptor, at 58 units, and assume that the units are feet (as suggested by the Taiidan corvette door), you end up with the interceptor being 17.6 meters long - the same size as the Hw2 interceptor.
The technical descriptions also match this as well. The Kushan heavy corvette is 102 units long - once again, longer than a football field if this is measured in meters. But the technical description in the guide describes the craft as a 2 seater, and the hull is so jam packed with electronics that the only way for the crew to get in is to have a technician remove the armor plating for the cockpit and then reseal it from the outside. Look at the size of the cockpit on the model - it would have a windshield 30 feet wide.
Considering that comments from the original Homeworld team envisioned the strike craft being the equivalent of modern fighter aircraft and used them for their inspiration, this isn't feasable and puts the final nail in the coffin that HW1 units are meters, as far as strike craft are concerned. Feet are much more feasable.
How then, does this stack up with the capital ships? There's no real way to determine if this is correct with capital ships. If the units are feet, then that makes the Taiidan carrier, the largest playable ship outside of the mothership, 503 meters long (1651 units) This seems well and good, but if fighters are 15-20 meters long and corvettes are 35-45, there's no way the carrier would be able to put 50 fighters and 25 corvettes within its bulk as most of the model is the cavernous empty docking bay, large enough to launch frigates.
If we also use the feet scale, this puts the mothership at just over 2100 meters tall (7015 units), which is smaller than an Orion stood on end. Here's where it becomes obvious that the unit scale for capital ships of frigate class and larger simply cannot be the same scale for strike craft.
The Orion has a crew of 10,000. The mothership held at least 60 times that, even if most of them were in cryogenic stasis. Cutscenes in HW1 show the cryo units together, and it's obvious that each pod takes up some space(both in the cutscenes, official artwork, and the Cata guide). If we go off that cutscene and give each cryounit a minimum estimate of a 3x3x3 meter "bubble" around it at 27 cubic meters, then at 100,000 people per cryo tray, you have each tray requiring 2.7 million cubic meters just for the containers, not including anything else such as the mass of the tray itself, or any machinery, air tanks, reactors, coolant pumps, thickness of construction between walls and decks, etc.
Going off the figure we now assume is true for strike craft, each cryotray is 430 units wide, 1167 units long, and 105 units long. So that's 131m wide * 356m long * 32m deep, yielding a total interior space for the bounding box of 1.5 million cubic meters. Keep in mind that the cryo tray model only takes up half the bounding box and you now have 750,000 cubic meters, assuming the entire tray was packed with cryo tubes and no part of it was dedicated to other machinery or fusion reactors, etc. It's safe to assume that one cryo tray can hold a quarter of the 100,000 people they are reported to hold.
If the scale of the HW1 units is meters, however, you end up with a cryotray with 26.3 million cubic meters - much more ample space, but possibly too large. If the scale for units of capital ships is, say, 2 feet per unit, then you have just under 6 million cubic meters. If each unit is half a meter, you end up with 3.2 million cubic meters of space.
So basically, what this means is that for the cryotray and all subsequent ships that have a relationship to it (The mothership requires the side door to open, and all capital ships need to be captured or released through this door along with some frigates) is that the scale of HW1 units for these ships is between half a meter (low side) to a full meter (high side). And here is where the Rule of Cool comes in.
What works with scale and fun for an RTS doesn't necessarily translate well to a space sim or an FPS. Many games, such as Risk, Starcraft, Civilization, (hell, I remember playing the old Earthiege mecha and RTS games, and those didn't scale accordingly) etc, use models similar to Homeworld to represent a particular unit more so than being an accurate representation of the unit itself.
Nobody believes that different building and units in Starcraft or Warcraft or any number of other RTS's are EXACTLY TO SCALE with each other - therefore, I don't see it the same way with the HW1 ships. The ship sizes are the size they are not because that's their ACTUAL SIZE, but because it fits a perfect visual balance for an RTS.
HW2 did a much better job of this - it artificially makes strike units bigger to make them more visible. You can turn this off with a command line parameter, and then the ships become so small it is difficult to select them without hotkeys or spotting the trail they leave behind.
So yeah, it then comes down to creative liberties within those two scale extremes for anything larger than strike craft. I've chosen to go with the larger side for non strike craft ships and have each unit be one meter each. This will mean that frigates are halfway between FS Corvettes and Cruisers, Capital ships are between corvettes and destroyers, and each mothership will be comparable to the size of the Colly/Sathanis.
While it's all open to interpretation, this is my take on the different sizes and scales, and how they can reasonably be put in the FSO engine to make a good, balanced game. A 2km long mothership with a crew of 600k+ is as unrealistic as 100 meter long strike craft.
A few ships will have their scales tweaked a bit further as far as they interact with other ships. Resource controllers, collectors, and the Kadeshi swarmers/fuel pods are all a bit off and will probably be scaled at a 2/3 or a 1/2 ratio.
This scale difference will make interactions between strike craft and capital ships more noticeable (the salvage corvette latching onto other ships, strike craft docking with support frigates or resource controllers) but overall it shouldn't effect gameplay mechanics too much.