Issue will present itself when people take too long at the vacs, so all vacs will be full and people wanting to vac will wait in your exit lane and block all traffic coming out of the washes. Size of the vac spaces doesn't matter, it's the location primarily and the few available vacs secondary that will cause jam ups on the wash exit. People will sit there and detail their cars, even if not using the vacs. They don't care about your business or other customers. You would almost have to have someone policing vac area and directing traffic on busy days.