I would probably try a 22x8 or 22x10. For your first props you probably don't need to go with the top of the line stuff at this point, although a good prop a short time down the road is a good investment. The book calls for 22x10 Mejzlik, or a 21x10 Menz Standard.
I'm just using a Zinger 22x10 for now, until I get some time built up on the engine. I do know the high-end props do perform much better then the cheaper props. I'm still in financial recovery from the whole plane, and building process, so I'm trying to save up for a few of the better props. Also, with more expensive props you want to be very sure of your landings, and ground handlings. You might even try a 20" prop, although it might tach to high for this engine. One other thing to think about is ground clearance. What's the clearance like on the DP Extra? I know the Midwest Extra does have enough room for a 22, as should the DP Extra, it's something to think about though.

Question here now....is this the plane that you were asking about engine mount not being centered? I'm wondering if it is, as the mount itself in this plane probably would be offset some for thrust. Actually if you look at the whole mount/firewall it should have some thrust angle built it. The mount is offset to compensate, so the crank comes out very near the middle of the plane.





Hope this helps.