I'm a bit over 5'10 and I ride a 54cm c-t. It was the size I felt most comfortable on after trying both a 54 and 56. You shouldn't base the size solely by stand over but it sure is nice having ample room for the boys, so it's still a criteria for me.

Best thing is to go and try few different size bikes. If you are 6' I'd suggest you try a 56cm see how it feels and if it feels too small go for 58cm. It's all just speculation until you actually try it.