OP - I'm in the same boat. Seating at 11-12 feet and looking at 106, 112, and 120".
I strongly recommend what others said, buy the projector first and play with different sizes for a week at a time.
To answer your question, I don't think 120" is too big. I was at 144" from 13' and that was big. But my wife loved it. I actually had a friend that vomited during Casino Royale due to motion sickness- no joke.
As I type this, I'm watching a movie on a 4x8 sheet of foam and I'm pleasantly satisfied. A 4x8 sheet is 48" tall, which yields a 98" 16:9 screen.
Keep in mind, the smaller the screen, the sharper and brighter the image should be. I'll probably go for 112" myself. That'll feel big after watching 98".
I used this calculator to figure out sizes.