I'd start with a 12x6 and an 11x7 on that plane. Those should let your engine unload in the air and make good power, and should make your vertical performance as good as you will ever need it for big loops and hammerheads. It will probably be readily apparent which one you like better after a couple of flights. I ran a 12x8 on my Super Tigre G90, and felt it took good advantage of all that torque. I think it would be a bit much on a .61 though.