My testing has shown the best prop on a .91FX for ultimate thrust is APC 15x6 (should turn around 10,500, if I remember right, and pull about 12 pounds). A little less thrust but instant spool is SK wood 15x6-my choice for 3D on a 8+ pound plane. It sounds like you have plenty of motor for the plane and don’t need the best thrust? If you are around 7 pounds RTF and have a good running 91FX, anything from a 13x6 to 15x6 will work. I have not found a thrust benefit to 4 pitch props 16 inches or larger. I did have a 16x5 wood that worked well a SA100 and FL330.

Tianci is right; load the .91FX with anything more than a 15x6 and it falls on its face.