Since this is going to be a detached building, I would encourage you to think about running plumbing. At the very least a laundry tub sink and a commode. Having water and a bathroom in my shop has really made a difference for me.
Also think about heating and cooling. You'll get much more use out of it if you can use it year round.
I have 10' ceilings and have never felt I should have gone w/ 12'. Haven't bumped anything on the ceiling yet. If you decide to go w/ some type of heating/cooling, those extra 2' will equate to 3,000 cubic feet of space.