Some suggestions.
- think about utilities (water, computer, phone, sewer)
- rough in the plumbing for a bathroom. Doesn't cost much, and you've got if you want it in the future.
- have plastic sheathing installed before they pour the floor. Keeps moisture from coming up.
- locate all the utilities in one spot. Makes for a clean installation and allows you better use of space.
- scissor trusses. Best thing I did! Opens up the space, allows more useable head room, and doesn't cost that much more to do. You can install floor trusses and have a loft for storage.
Here's a pic of the barn/shop I built in 1992. The barn measures 40' x 48' x 10'.