MINIMUM depth or length should be 50 feet since no coach is longer than 45 feet.
Minimum overhead door clearance should be 14 feet. A 50 foot deep bay would allow the largest coach owner to keep the tow-bar / hitch attached and still be able to walk around the coach, even with the door shut.
I store our coach in a 50x15w - a 20ft wide would be nicer for opening the slides all the way (but i won't pay 100 bucks more per month for the extra 5 feet).
I wouldn't install anything less than 30A service (50a would be nice fi you think folks might stay in the coach / apartment) ;) Water & Sewer would be an added benefit as you've mentioned.
If you build it wide enough you might consider adding a drain to the floor - then you could wash cars or the coach INSIDE the building (and dry them or hose down the concrete floor).
I love having the enclosed/private bay - it means i can work on the coach (change the oil, perform mods) and i don't have to necessarily complete my work and put everything away - i can come back and finish it later (waiting on parts) or when i have time.
