I would go as wide as possible. I just built a 3 car garage with 3, 9'(W)X 9'(H) (I have vans with ladder racks)garage doors and only went with the 36' wide..........I MESSED UP!!!!!!! My doors hit the walls when I open them.
I have 16" from the corners to the garage doors and 32" between garage doors. Example= 16"+9'+32"+9'+32"+9'+16"=35'+ my 2"x6" walls = 36'. I didn't have much say in the matter though because the city wouldn't let me build it any wider (Needed a variance to get this one done) If you dont go wider make sure you move your garage doors at least 24" from the walls and then center your middle door from there. Good luck.

This is one of those questions that's really hard to solve on paper. When designing our garage we went out into the driveway (because our current garage is too crowded to get a second vehicle into) and mocked up the garage situation with the vehicles, and some ripped open cardboard boxes for the walls, propped against chairs. Opened up the car doors and measured the space.

Found out we need and want more space than we thought. And our builder keeps saying we're spending too much on the garage. That's okay. We're only doing this once and we're paying for it. I say oversize it. And allow for wider vehicles in the future. Space to move around in is NOT a waste!