You have to go 24x24, but you can also go up. If you dont need a lift, I'd go the 24x24, with a second story for parts storage. It's the parts that get in the way. Use the side wall to make the bench(s). Becuase a ton of work is done on the front of the car (think about it) and you'll want to be able to putz around there. Not shimmy between things. Leave a generous hole in the ceiling with a hinged door, and have a pulley type lift thru the hole. You can also use fold down stairs or a ladder to access the same hole. Then, insulate it well so it stay comfy no matter the temps. Account for more outlets and lighting than you think you need, make sure there's enough "room" on each breaker for a high amp draw (like use more breakers and circuits) so you dont pop them all the time. I wont bother without a 200amp service to the panel either.