Baseships are 1500 kt in size and stand at the top of the hull tree. They are 50% larger than the Dreadnaught hull, which is the second-to-largest ship. They're slow (only 2 engines allowed) but extraordinarily powerful.
I just finished a game in which I was using solo Baseships with onboard shipyards along with some cargo capacity, quantum reactors, and a lot of fun extras. They didn't rely very much on their own firepower but, rather, had enough fighters and troops on board to deal with things. They didn't carry large numbers of either, but the space yard let them build reinforcements fast enough that combat losses weren't draining their effective strength.
Sweet!
Other nifty things in the full Version include solar sails (bonus to movements), cloaking (be invisible!), computer tech that gives bonuses to production, and bigger and better Versions of just about everything. Lots of fun stuff! I've still not fully explored everything it has.