Colossus : 6km, 30000 personnel.
This would mean that, if all was equal, the Peacemaker would be 20 km with exactly 100000 personnel. One tenth of a million.
Now if it was 10 times wider...

I guess it's possible, the Colossus isn't exactly wide. But to build something that big? Ugh...
Let's work it out. I know this is bodged because structural differences would change building style, method, order etc., but anyway:
Colossus - 20 years. Multiply that by 10/3 for added length, then by 10 for added width. At the GTVA technological level when the Colossus was built, the Peacemaker would take roughly 667 years. Ouch.
Smaller ships are
obviously the way to go. Well, yeah. Or something. Then again, my calculations are so heavily bodged I wouldn't be surprised if a GTAA Peacemaker popped out of the ground to say hi in a few minutes.