It's true - no matter what the size the shop - it will be too small sometime. So the building budget is probably the number one concern. A 12' wide bay is a pretty good width for one vehicle. A 40' width gives you three wide pretty well. Length is minimal at 40', and the longer the better. Especially if you want to put a trailer inside. We have a 40' x 78' area, and we can comfortably work on four large road tractors with plenty of "elbow room". Or we can back a trailer inside in the center for loading/unloading or servicing. There is a single 12' x 16' high overhead door for access. It's fine for a hobby shop. I would consider the height as very important. Shortly after we moved into the shop we now have, we had the opportunity to get a brand new four post bridge crane. Luckily we had the required 20' ceiling height. Everthing inside is now moved with the crane and a forklift. And we're real spoiled. So consider how you are going to move the pieces you need to move. Everything on these trucks is heavy. Moving parts and big pieces conveniently makes the job a lot more pleasant. You might also consider what you are going to do in the building - painting, welding, running engines, etc., and account for the special conditions as best you can. The air gets pretty thick quick when firing up one of these old diesels. Venting should also be a concern. We don't have any, and I sure do wish we did. If I were to design and build a smaller shop for one truck at a time, I would think about something between 28' and 32' wide and a 16' eave height, at least. 40' would be the length, if space allowed. Then I would put open side "lean-to" sheds along each side of the building, to help with the too tall look, and to gain the covered storage space outside. The lean-to's could have a 10' eave height and about a 12' width, which would allow most B model size trucks to park under. Certainly would allow any car, pick-up or tractor to be stored there. And great firewood storage. And boat storage etc, etc. Just some ideas - Good Luck with your project. Paul Van Scott