Rob, I have found on my Barnstormer with OS25LA that a Zinger 9x5 works great for my combination. Good as any place to strart. 9x6, 9x4 will also work depending on all your planes fuel, weight, and altitude that you fly at.
I'd get what ever is avaliable the quickest, and quickly go flyin.
Have fun, working out the right prop is part of trimming a plane.