I've been working on this for a while, with extrapolation based on the given ship crews and the crews of modern navy ships. I decided to add some extra length to account for larger engines, phase cores, armor, etc, which a starship would need, but a naval ship wouldn't.
Oh, and I ignored the fact that they'd be all equal lengths, since they're all the same length in game. I took that as a balancing property of the models.
Here's what I came up with:
Kol, Dunov, Marza, Sova: Around 300 meters
Akkan: Around 350-400 meters
Radiance, Halcyon: Around 300 meters
Revelation, Progenitor: Around 350 meters
Rapture: Around 350-400 meters
Kortul, Vulkoras: 400-500 meters
Skiranta: 500-550 meters
Antorak: 350-400 meters
That's my guess for them.
And that's pretty small, compared to other sci-fi ships.