Pick up a copy of The Complete Trailer Sailor by Brian Gilbert. Tons of info and how to's. But most of all (at this point) he includes specs, sketches, photos, pros and cons for about 50 boats from 15 to 28 feet. It can help you figure out what you like, what you can live with, what you can't live without. Will you be cruising at all? If so, I'd go as big as your car can haul and look for head room and head room.