You asked what's considered a large valve head?
http://www.scrappydogscooters.com/12...rformance.html
30mm intake and 25mm exhaust. Those are considered BIG valves for a 2 valve head. (actually larger than most "big valve" heads)
http://www.scrappydogscooters.com/12...rformance.html
23mm intake and 22mm exhaust are considered BIG valves for a 4 valve head. Considering the "other" 4 valve heads use 21mm intake and 19mm exhaust.
I think it answered your Q quite well.....