Prop Rule of Thumb to Minimize Torque Roll Tendencies
Whenever you are building a pusher prop, jet type plane, to minimize the effects of torque roll, always keep the prop diameter 1/3 (33%) of the wingspan or less. With a 28" wingspan, a 9" prop is 32% of the wingspan and would be a good choice. A 12" prop is 43% of the wingspan and will generate a large amount of torque roll.
On the larger models, if you can keep the prop down to 25% of the wingspan, there will be almost no discernable torque roll tendancies. My 166% F-18 has a 48" wingspan, and I am running an APC 11x8.5 prop on it. In this case, the prop diameter is 23% of the wingspan, and it has almost no torque roll coupling.
Lucien