To make race-specific ship sizes, give them a tech prerequisite that has its Racial Area set to one of your defined racial areas in RacialTraites.txt. Just like you do for a component
But remember, if one or more of an item's prerequisites (direct prerequisites only - not prerequisites of prerequisites) is non-racial, then the technology can be traded or stolen by other races! (That's how some of the special techs in the Star Trek Mod are stealable
)
E.g. if Gelf Frigate has prerequisite Gelf Technology which is racial, Gelf Frigate will only be buildable by Gelfs, even if Gelf Technology has prerequisite Physical Paradigm or something... but if Gelf Frigate has prerequisite Gelf Technology and Ship Construction which is NOT racial, then Gelf Frigates can only be built by Gelfs UNTIL they trade the tech away or someone steals it, either by espionage or ship capture.