91 fx prop
The rule of thumb, or old wives tale, as I have been told, is that 1 inch in diameter equals about 2 inches in pitch, for load purposes. Thus, a 14x6 and 13x8 have about the same load, and are supposedly the ideal props for the 91 fx. Extrapolating from there, 12x10 and 11x12 would be a good place to start, if you wanted to add pitch to the mix, without adding load. The 12x9 and 11x11 would allow more rpms, which is also good. I suspect that a 13x11 would be too much prop for the 91.
Your results may vary.
Paul