i suggest no smaller than 22' wide X 24' long with a 16' wide door. you can do vaulted ceilings inside for hetght if you plan on installing a lift. the slab should AT LEAST be 5" thick @ 3000lb psi pour and make SURE those assholes put rebar in there and check the slump when pouring the cement. if you plan on installing a lift check it's specs to see if the area on the slab it is to be installed on meets the manufactures spec. if not, the foundation could crack and your car could come crashing down. ac/heat , plumbing for wash sink (you'll need it) spend the extra $$$ for insulation and drywwall. seal or epoxy the floor after it has cured about 27 days. wire it with several circuits for expantion and multi-equipment use. the avarage garage build is around 10K. for guys that plan on spending a lot of time out there, it may be a bit more$$$. do as much work yourself to save $$$.
