INMH, you build the barn to take the largest MH on the market....just in case. Also, if you decide to sell the property you would hate for it to fall through because the barn wasn't large enough for the buyer's RV.
Second is to install the largest door that the design permits.
How wide is somewhat secondary and is an individual design based on what the perceived needs are for additional floor space.
Mine is 20 W, 48 L, with 15' sidewalls. Door is 14 H X 16 W. Interior hgt is 16' to the trusses and 17+ to the peak so plenty of room on the roof
This size allow plenty of room for the coach with slides extended and a workbench. air compressor, and the such. 50 A power, good lighting and several outlets. No dump station is my only regret but I have alternate methods of dealing with waste.