The headaches may not be due to the screen being big. Generally, the headaches are a result of getting used to another stimulus, whether it be the "sudden" change in screen size (going to a much bigger screen or a smaller screen than what you are used to), a brightly lit screen in a dark room (which you might not be used to), or some type of display artifact (plasma, LCD, and DLP each have their own types of artifacts). Sometimes you get acclimated to the change after time, and sometimes you don't (could you ever get used to listening to a "bright" system?).
The "accepted" values ave already been posted. 3:1 for TV viewing, and 2:1 for movie viewing. You could even go a step further and use 1.5:1 for high definition movie viewing (720p, 1080i, 1080p). But 2:1 or 1.85:1 is a nice distance to shoot for with movies.
But I can help but comment on something here. I do see a number of audiophiles seem to take pride in bucking the trend with videophiles and shooting for longer viewing distances and smaller screens, and being proud of that fact. That strikes me as odd. It's almost like a reverse snob thing. But then again one also sees similar parallels with non-audiophiles. One complaint from that side is that audiophiles listen to music "too loud." That's not meant to be overtly conclusive, but rather just a passing observation.
My point of reference.
-In the theater I sit 1.5 times the screen width back from the screen (about row 5 or 6).
-At home, I sit 1.85 times the screen width from a 800x600 front projector
-watching TV at home, I sit about 4:1 viewing distance (bedroom). it was about 3:1 when we had a bigger TV, which was sold for financial reasons.
-I listen to classical music between 60-72dB. And rock 68-76dB.
It really seems odd that a 42 inch at 11 feet, which is over 3.5:1 viewing distance, caused you that much problems. Your 34 inch at 11 feet is about 4.5:1. If anything, I'd think it was due to some other stimulus (alcohol, food, unfamilar setting, poor source material, display or display technology artifacts, too bright, too dark, etc). Or just the fact it was a sudden screen change in side (if that's it, you will get used to it after a while). Or maybe just laugh all the way to bank with a smaller screen. I dunno.
Aaron