A 22x10 would be a good start.
Use my [link=http://mvvs.nl/prop-power-calculator.xls]calculator to try out different rpm/prop combinations[/link]
In there are options to choose your flight field elevation, and prop type. If you have noise restrictions, check out the max rpm for noise.

@ Bob Atkins:
If your son needs the calculations to update his code for ThrustHP, he is welcome to it.