I am getting ready to build my first (probably) gas powered plane, a 1/4 scale BirdDog. If I use a scale diameter prop, and I would like to, that's 22.5 inches. I am also getting a US-41 engine for practically nothing (NIB $175) with wiring harness and Pitts style aluminum muffler. All it's ratings are using a 18-6 to 18-10 props. Would it handle the larger diameter, and if so what would likely be the best pitch. If not a 22 prop, how large can I go?

The biggest prop you can run on this engine is a 20 x 8. It likes the 18 x 8 & 18 x 10 also.

i seen a guy with a 1/4 scale balsa usa tiplane with one in it. he was using a 20x10 zinger. later he went down in pitch. he only flew at half throttle. you'll have no problem running a 20x8 or 20x6 for sure.