I guess it depends on how elaborate you want the mirror to be. If you're going for a modern minimalist look I'd keep it to one inch or less. If you want something more traditional then you can go beyond that. I'm looking at a picture I have in my living room. The frame is about 1.75 inches wide and 1.75 inches thick. It's definitely not overwhelming but it's pretty traditional. I think contemporary frames that are that thick and wide detract. When the frame is thinner it looks sleek and sophisticated. But that's just me. My "pop art" prints are all in a plane black 3/4 inch frame.

Lotteryticket, that makes sense. I have a plain 2" wide frame on a large living room mirror and it's neither fish nor fowl. Definitely not traditional in frame styling but a little too wide and heavy to be sleek. Now I understand why. It's okay, but next time I'll know better.