Here are more ideas for ship sizes, borrowed from MOO2/3, Stars!, my imagination, and other sources (sorry if some have been suggested before):
REGULAR SHIPS
lancer
cutter
scout
heavy cruiser
dread cruiser (serious, they were going to use this in MOO3 but changed their mind!)
superdreadnought
titan
behemoth
colossus
leviathan
nubian
SPECIAL PURPOSE SHIPS (don't bother including them in any standard, but they might be cool in a mod)
privateer (built in cargo space)
rogue (see privateer)
galleon (see privateer)
mini colony ship
midget/mini/(regular)/maxi/ultra miner (how about giving these mining ships a special robominer mount that reduces its size by 50%?)
fuel transport (built in supply storage and solar supply generation)
super fuel transport
mini/super mine layer (for special racial traits like "Space Demolition" - special mount reducing minelayer component size)
BASESHIPS
doom star
death star
death egg (OK, maybe if you are doing a Dr. Robotnik shipset with eggmobiles as fighters
)
STARBASES
orbital fort
space dock (built in SYD ability?)
star fortress
ultra station