I would say that the smaller the blad the less torque for a givern TSR. To get the best out of the Ametek you really need to produce a RPM to power out put (in watts) graph. From this first graph you can find the point at which you get the best compramise i.e. the watts goes up linear with the rpm and peaks, this would be around the point at which the best fit line starts to flattern off. Find this and read of the RPM. You also really need to find the average wind speed or at least asume one. Using the wind speed you need to build a prop that turns at that rpm you found earlyer in you average wind speed thus getting the best power at your average wind speed.
You can get a programme that determins the cross sections using formulars that i do not know of the top of my head. Produce a blade from the programmes information, and then you should have a good prop thats apropriate for your Ametek. I think this is right, please anybody correct me if i am wrong??
The only other issue i can see is mounting such a set of blades to the Ametek, the bearings on these motors are small and not really up to the job of having a woodern prop as the thrust loads and vibration will make the bearings wear out fast. I have use similar motors and had to install a secound lot of bearings to handle these forces.
I think thats about it, hope this helps Ben.
Regards Jonathan