I just got back from the field and a member had a us 41 swinging an 18x10 Xoar on a 1/4 scale super decathlon.(98" WS). It did fine with that prop on it. I know some people really like a scale prop escpecially on a contest plane but, very rarely due they actually fly with a scale prop. At least from what I've seen. They are mostly just for staitic display. Look at most of the warbirds for example. During static display they all have beautiful 4 and 3 blade hamilton's hanging on them. But, when they fire the engine up for flying they have a 2 blade wood or carbon fiber that is best suited for the engine they have chosen.
I would guess that something in the 17 -19" range would be a scale size. Just my .02
P.S. Xoar makes some good looking painted props that might fit your need, I believe they are called swords.