My usual make-up:
Tiny vessels:
I never build tiny vessels. No need to do such a thing.
Small vessels:
Defense Ship - 1 engine, triple defense system(means three of each if agaisnt colorful enemies, if not, focus), lots of guns, does not deviate from it's designated planet's orbit
Guerrilla Ship - 4 engines, single defense, great support, the rest is guns, takes out trade routes, starbases and vital enemy targets
Medium vessels:
Assault Ship - 2-3 engines, dual defense systems, good Support, the rest is weapons, good for pressuring and maintaining a battle far from your worlds
Defense Ship - 3-4 engines, single defense systems, no support, lots of weapons, primarily used to intercept enemy attack fleets
Large vessels:
Main Battle Ship - 3 engines, dual defense systems, great support, lots of guns
The Really Heavy Guns - 1 engine, single defense system, good support, zomfgingly huge number of guns
Huge vessels:
Heavy Battle Ship - 3 engines, dual-triple defense systems, good support, a heck of a lot of guns, travels in fleets of Heavy Battle Ships, or bolsters Main Battle Ship fleets
Flagship - 1-2 engines, triple defense systems(if I'm Yor, I'll take quad even), huge number of guns, can take down whole fleets by himself, usually travels in a fleet with The Really Heavy Guns, I never have more than one Flagship at the same time
And that's what normally takes down the galaxy.