A list of valid abilities that can be used in the VehicleSize.txt can be found in chapter 16 of the modding tutorial:
http://se4modding.spaceempires.net/ModdingTutorial.html
I've used some of the vehicle launch abilities on hulls or movement bonuses, but there a lot of other abilities that can be used to do some wierd stuff - check the ability list from the tutorial site above to get more ideas.