The tallest you can be going down the road with out special over size permits is 13"-6", I have to assume 14' would be fine.
Me personally, I would want a wider door... Most campers are 102" wide and that doesn't include the awning roll, which would put you at roughtly 9' wide. Backing a 9' wide camper into a 12' wide door gives you just 18" on each side... Certainly duable, but will be tough to put the camper anywhere but straight in in the center of the shed.
I am building a new building next summer, but my current shed has two sliding doors and a 20' opening... its a breeze to back thru even on an angle to put the camper along one side.
2014 RAM 3500 4x4 CCLB Dually, Cummins / 68RFE / 3.42 / B&W Patriot 16K
2016 Coachmen Chapparral 360IBL
SOLD - 2012 Salem Cruise Lite 281BHXL
SOLD - 2005 Dodge Ram 2500 4x4 QCLB Cummins