My wife and I built 2 sheds. The one on the left is for the mower and garden tools, the big one is for wood working and stores motorcycles in the winter. The big one is 12 x 8 and is a good place to hide during a rain storm.
Windows are from Craigslist (cheap).
Building codes around here allow up to 120 sq ft without a permit - that's 12 x 10. I chose to build a 12 x 8 because the footprint is exactly 3 sheets of plywood. The sheds are set on cement pier block with adjustable height screws for holding the floor joists. This way you can put down the pier blocks and then adjust the height without having to move all the cement blocks individually. The problem with this is that the sheds are not considered a permanent structure unless they are cemented to the ground - so if I was to sell the house, the sheds would not add value.
My wife and I built the shed walls on our back patio and stood them up on the shed floor. It was pretty easy for us to do, and it saved a ton of money over having a completed shed delivered or built by professionals on site.