ORIGINAL: OzMo
If you go up in lenght 1 size go down in pitch 1. (the opposite also works)
10 / 8 should be swappable with an 11/ 4 or a 9/ 7 roughly.
Up 1 in length and down 2 in pitch
If you're using a 10X8 now, an 11X6 would be a match as far as load on the motor. A longer prop with a shallow pitch will give you less speed but more thrust. A shorter prop with a steeper pitch will give you less static thrust, but more speed.