I built a 30 (wide) by 50 (deep) pole structure with a 13-ft ceiling. With a concrete floor. I never finished it (no doors, it was open on one end). But here's a couple of thoughts -- It was affordable and a good rugged start to what could have been a really good shop. BUT I did learn that 30 is too narrow for three bays. Mike B's suggestion of 15-ft wide work bays is spot on. but 10-ft wide parking bays might still be a touch tight depending on the size of vehicle you park in there. I could easily park three trucks next to each other, but working around them, or even opening doors, was tight. Big trucks got to be a problem making sure mirrors didn't line up with the wall uprights or each other, etc or the doors couldn't even open at all. Look at modern aircraft hangar designs -- more width, less depth, no "buried" vehicles.
Not sure as I never tried it but 30 deep by 50 wide would probably be the better arrangement (I was constrained by my location). But if you have to go deeper rather than wider, the "drive through" ability is critical for maintaining your mental health.
The other thing I learned is that bigger is NOT always better.
