I agree that even at 12x12, things'll be "cozy" inside. That's OK. Keep in mind that this will be a general purpose hobby shed, not just a wood working shed. Even 12x8 would be plenty big enough for building 1 meter wingspan bungee launched RC gliders. And for the electronics projects I do, I only need enough workbench space to do some soldering with a 3rd hand (for electric guitar pedals). If I need more room than the shed offers for wood working projects, I'll just roll the table saw down a ramp and do my cutting outside in the sun.
I had a similar experience with running electrical power to a shed. It turned out that I could build a 12x12 shed without a building permit as long as I did not run utilities, but getting a permit opened a whole new set of requirements that forbid me from having a shed at all. And when the nosey old woman next to me kept calling the building inspector on me, it was suggested that I could run an extension cord out to the shed anytime I wanted as long as it wasn’t fastened in a permanent manner. 

My advice? Don't do this. If you have jointer and a planer, use them. If you don't, seriously consider using dimensional lumber that has already been planed and sanded. If you are going to try to clean up construction lumber by hand, using a hand plane is a lot faster and more pleasant than using a belt sander. Except, of course, that to do a good job of planing a board you need a solid bench to hold the board, and you don't have a bench, yet.

In each Practical Woodworking Class, the daily mix consists of intensive lecture, demonstration, and student practice lab time.  Lecture periods are interactive, informative, and intensive but are also an enjoyable experience.  Generally, each joinery and fabrication process is demonstrated in several ways to reach or achieve the same results.  For example, processes are first demonstrated with the use of hand tools. Next, the same joint or procedure is done with simple hand held power tools (such as the router). Then, the entire process is demonstrated with stationary equipment such as the table saw, drill press, and mortiser. Students choose whichever production method works best for them and is in keeping with their plan to equip a home workshop and/or around their existing home tooling.

Our Lexington model was designed to match many traditional New England homes, in Massachusetts and beyond. The steep pitch of the roof is built to withstand sometimes-punishing New England snow loads and complement today’s home design styles. Post Woodworking can match most vinyl siding colors, so that your shed color will coordinate with your home as well. And as is the case with all of our sheds, interior solar lighting is available to light up the night.