Built it big enough for your need, Put alot of cement in the floor say 6" at least and put rebar lot of rebar in it so it will not crack, and have some drain in it, even if it dump into a dry well, Lot of lights, and a big fuse block to hold alot of circuit breaker, and if you are going to finishes it off later date then use Fire proof sheet rock, make sure you got alot of outlet(mine are every two feet) heating system so you can keep warm to work on your car without freezing.
decided what you are going to do about an Air Compressor, and then pipe it around the garage also have a outside Outlet for Electicity, and Air, also motion detector Light on the out side of the garage, and if you are storing your Valuable Mopar car in it then I would install some sort of an alarm system, also in stall a Phone line out there. this spec is for Cold weather State since I don't know where you live at the moment. More to come after I think about what more I can put into your Garage for you