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.
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.
The Practical Woodworking course at the JD Lohr School of Woodworking is a complete and comprehensive woodworking course held in our shop on the 13-acre farm in southeastern PA. This is total immersion woodworking for ambitious, interested adults. With class size limited to 11 students and 3-4 instructors in each class, all needs of every student are met regardless of experience level. If you are self taught with no formal training as a woodworker, this course will quite literally change the way you work.