Always overbuild, you won't ever regret it! Watch the width, trusses can only span a certain distance without using a support in the middle, I think 26 feet was the max. Pour a 3-4 foot deep frost wall to prevent the cold from penetrating into the floor. You can even put styrofoam insulation under the floor. Lots of rebar, or kittiy-hair, in the poured floor, pinned and tied to the walls. Saw cut the floor or use zip-strips to help control floor cracking as it cures. Ideally, you want to pour the concrete on a cool (60 degrees), damp, cloudy day. This will slow the initial cure and make it easier to finish it. Don't try to do the concrete work yourself, contract that out to a professional. Keep the footings a square as possible, mine was off by 1/4 inch out of square, and it followed all the way to the roof sheeting, everything was out by 1/4 inch! Not much you think, but wait 'till you put the drywall ceiling up. That little 1/4 inch adds up. Outlets, outlets, outlets. You can never have to many. The same goes for lighting. 220 service is a plus also.