I agree with Bill on the size! I plan on doing the same thing and will build the barn at least 40' wide and 60' long and with at least 14' walls and a 14' high door. I'd also like to have a door at each end so all I have to do is pull through to drop the unit or retrieve it when ready to head out again! I do want to be able to open up the Monte when inside and do some work on it, both in and outside the unit. I plan on putting the adequate power set up in it just incase I want to get another rig that has the 50 amp setup. Mine currently is a 30 amp unit. I also plan on setting up a full hookup setup so all can be used IF NEEDED!
Could be Rondo's Man Cave if need be!
I would like to put some sort of heating unit in it also, so I can work in relative comfort in the winter if need be. The blow on foam insulation seems to be the best so far and then run plywood up the walls at least one sheet high(8 ft). This may be somewhat of a dream but hopefully that dream will come true over the next year or so. Probably won't have any money to go out with the unit after that!
Keep us informed on what you do and post some pictures also when you get it finished.