I wouldn't want to go near anyone with a girth above 5". I'm rather petite when it comes to sex, and it would annoy me having trouble fitting it in my holes, haha. I know from my experience of sex toys that 5" hurts, but it can be in a nice way (although, at the mo, anything above 4.5" is still leaving me sore for a good few days)...more than that wouldn't even fit, though. Below 4" would probably bug me a bit, though, unless they had some nice curvature going on.
In terms of length, as long as it's above 4" long (and as long as bigger lads don't mind not having the entire shaft go inside), I would imagine all would be well. Different positions work for different sizes, after all.
Perfect is around 6.5" long and 4.5-4.75" circumference...but then I may be biased as that's roughly what I'm used to (I forget what my boy is girthwise, somewhere around the 4.8" mark, I think, which right now is a touch bigger than I'd like >.<).
All that said, though, I don't think size matters that much, as penis in vagina sex is probably one of my least favourite things in the whole sexual repertoire (don't get me wrong, it's good...but there's so much more fun to be had with other things). I don't think I'd ever break up with someone based on penis size, but I've never been in a situation where I've been unhappy with that.