I have an RV-6A with an O-320. Initially I had a 150hp motor and a 77" prop, which was a good match. I then upgraded to a 160hp motor and had to get the prop re-pitched to 79" to keep the rpm under control. That wasn't really enough as I was running an E-mag, and so went up to 80". Recently I have made some cooling improvements, installed an updated E-mag and new Slick harness and can now get up to 2640 rpm at low level, not that I stay there for very long. So 81" may not be too much. Bear in mind this is with a 6A. A taildragger might need up to an inch more. Clearly there is a compromise with climb rate, but at 80" I'm still getting >1000 fpm at max weight.
Hope this helps, Pete