It totally depends on the length of your legs, arms etc. Long legs = taller frame, and vice versa. And don't forget that the top tube length is rather crucial, too. You can only compensate so much for wrong length without getting into trouble with the balance and weight distribution.

I've found that some of the free bike fit programs seem to give numbers that actually fit well with what already feels right. If I remember rightly, this http://www.competitivecyclist.com/za...der=M&units=cm was one of them. As my proportions are not absolutely average, the fact that it came very close to my own ideals speaks in favor of it.

Apart from that, this guy is an old sage http://www.peterwhitecycles.com/fitting.htm and I think his attitude is better, in fact, than slavishly following a formula.