A good size is 26' deep by 24' wide.

The roof has a 12" pitch which gives you a good storage area in the loft.

I included a 20" x 16" x 10" deep stainless steel sink. This is handy for washing your dirty hands. It also comes in handy gardening. Since the inspector was concerned about gasoline going into the septic system, I had to run that sink to a separate dry well.

I insulated the ceiling with 8" and the walls with 3 5/8". I also have insulated vehicle entry doors. By doing this I'm able to heat the garage with a 5KW electric unit heater with built in blower. This is safe and you turn it on and off as required. I'm also able to cool the garage via a 7,000 BTU air conditioner. Both of the above work very well. Not sure if the field stone building has anything to do with it. For some reason the building interior never goes below 35 degrees unheated.

Make sure your ceiling height is 9'+ to accommodate a twin post lift if you ever decide to add one.

Consider your floor thickness for this as well. You may also want to have the floor steel floated. This makes the floor dust free and makes it easy to clean up spills.

You may also want to place a 2 inch galvanized pipe in the cement at the garage door entry where the door meets the floor. This insures a long term garage door seal.

Make sure you provide many electrical outlets around the perimeter.

At the work bench make sure the outlets are fed by separate circuits.

Make sure the lighting is on separate switches and circuits from the outlets. This lets you light up just a portion of the garage and prevents the lights from going out all at once. It also keeps the lights on when you cause a problem with an outlet circuit.

I also used a Bessemer one piece sliding staircase to access the loft. This lets you carry a 80 LB bag of cement up the stairs without falling through them.