Lots of good advice given here. When deciding on minimum width, you want to be able to open not only your slides, but your bay doors, and still be able to walk by. Some of those bay doors are longer than others. I would say 20 feet wide is a minimum that would comfortably meet these needs, although 18 feet would probably suffice.
As for length, you want to fit the longest coach made (which is 45 feet) plus have room in front for mirrors and in rear for a tow bar, and then be able to walk in front and behind. I would say 55 feet is minimum.
Door height 14 feet minimum as many new coaches are in excess of 13 feet tall. My door is 14 feet and I have a flexible radio antenna that harmlessly scrapes the door every time I enter or exit.
Will you want space for storage, a workbench, shop tools, or a car or boat? If you are able, make it bigger than you think you will need, then enlarge it again. You will be glad you did so.
50A hookup, water & sewer are great to have. What about a utility sink in the garage? Mine comes in VERY handy. If I did it again, I'd put in a toilet too.
Marc and Jill, Wellington FL
2013 Entegra Anthem 44SL