After racing XC last year, I wished I had a set of 20" and 22" rear tires. A muddy course where handling isnt as important would best be run on 22" rears, a faster course I would stick w the 20" rears. 22" fronts are as tall as I would personaly go with. 23" has too much sidewall going into turns w too much roll-over. Just an opinion of course!
I like my 23" fronts and 22" rears, but I don't see much mud. They did do very good in the mud fest in Havasu though. I have the taller tires more for all the rocks is the desert.