My father-in-law built his first shop about 10x12. Then he built his second shop next to the first shop, 12x16. Then he built an "annex" to the second shop where he stored all his scrap wood, that was roughly 6x12. Then he built his third shop, which was 16x32, a few feet away from the annex. Eventually he built an annex to the third shop, 8x16, to store things he no longer wanted in his big shop. He did lots of great work in there, and the string of buildings against the back property line was actually nice-looking, like a homestead back there. He died before he could build shop #4.
I think the moral is, build it bigger than you ever think you would want. Mine is 12x30 - 12' was the max width due to setbacks. My wife wanted me to build it 12x40. I wish I had.