Check your zoning laws. See what you're allowed to get away with. I.E. In some places installing a bathroom makes it "liveable space" and is taxable as such.
If I had my druthers here's what I'd do. Make it at least 1.5 stories tall (preferrably 2) and double the car length deep. Get a working lift at one end and a four post storage lift at the other end.
Add plenty of working counter top, and storage (I like the chrome baker's shelves). Put in a mammoth air compressor and run hardline around the building with quick couplers. Heating & A/C are *very nice*. Make sure you've got plenty of outlets wired and *lots* of good lighting.
The nice amentities are fridge, coffee maker, wireless internet, couch, and flatscreen TV.