When you increase the diameter of a prop the load increases on the engine in an exponential manner. The load farther from the center of the crank has much more leverage against the torque of the engine. When you keep the same diameter and just increase pitch the load will increase about the same for each step of increased pitch. When you increase diameter, each step in increased diameter loads at a higher rate, as an example, the first increase may cost 600 RPM, the second increase something like 1000 RPM, the third, 1500 RPM. The numbers are just examples to illistrate what I'm talking about. Not even sure where you could get a 26x6 propeller, Zinger makes a 26x8 that might load like a good prop at 26x6.