Make it as large as you can fit and afford. We have a 42x96 farm machine shed with doors on the long side. Four bays each with a door 16' high. When we went from a 40' coach to a 43' we had to push one bay out 8' so the new coach would fit with access to the engine from the rear. In addition the width should be wide enough to allow access with slides extended. Remember, you're going to need room to store those many things you have in the coach and may not want to keep with you on the next trip. Also tools, oil, antifreeze, and even cleaning supplies. Although we have 50A service to the Bus, we don't have a drain for sewage or water supply in the barn. We can dump right outside of the building to a dedicated dump and can fill water from outside with an extended hose. Make it as big as you can and then go to the next size bigger. You'll never be sorry.