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...