First, let me say: very very cool. It's different, and it had to grow on me. My initial reaction was "what the flip"? But after a while, I started to dig it.
In the game Star Fleet Battles (by ADB), the Federation Police Cutter is about the size and roughly the shape of the TOS Constitution-class cruiser's secondary hull. The Burke-class frigate is about 1/3 ~ 1/2 again larger. The Saladin-class destroyer is another 1/3 ~ 1/2 larger yet.
See here for images:
Ergo, I would say this is at most a frigate but probably a cutter / corvette. That would give it a crew of about 100 to 125. (In the game, the frigate has a crew of 160, and the cutter has 100.) The smaller engines aren't a problem, as they are sized to the ship's lighter mass.