Legal maximum height for vehicles in the US (without oversized permits) is 13' 6", so a 14' high entry into a garage or pole barn should be sufficient for any RV unless the ground and/or floor is quite unlevel or uneven. Legal maximum width of an RV I think is 8' 6" plus appurtenances (things like clearance lights, mirrors, grab bars, and awnings), but you probably want the width of the door somewhat greater so as to make backing in or out less of an exercise in extreme precision maneuvering...doubly so if you don't have plenty of space around to line things up.