Rough attempt at scaling the Gatlantis double-deck carrier. I did it assuming the turret is the same size as the main ones on the Calakmul, with the Gostok as a back up. It looks to be the biggest ship in the series, around 1000 meters.
This of course means we'll likely not get a 1/1000 model.
sigh 
Anyway, I did another scaling using the original double-deck carrier. I tried to match up the width of one of the runways with the width of the Nazca's runway. This would put it roughly at 500 meters.
I also stuck in the Guipellon and Gelvades. Makes me realize the Gamilas carriers are actually a bit small compared to the other capital ships.