Agree with the above.....I am 6' and ride a 23.5" which is about 59-60cm...The top tube length plays a big part too, althugh you can help this along with shorter/longer stem.....
If you are nearby, you could come over and have a play on my bikes to check for fit......I have 8 to chose from!
The only smaller one I have is a cyclocross frame which is 56cm......
I'm near Stansted airport