I'm 5'9" and ride an 18" P20. They aren't the biggest, i could ride the 19" without problems. I've got it set up with a 120mm stem and XTR layback post and its still quite short.
The geometry is here:http://www.oldmountainbikes.com/catalogs/ritchey/1997/ritchey1997_08.jpg
The rest of the catalogue is there aswell. Hope this helps