Decide if you want a fixed of folding style, and contact the manuafacture with you boat type, motor, trans ratio among things you will need, and they can tell you the prop diam and pitch needed. SWAG would be a 13-15" prop. Two vs three is another debate per say. The size etc is the same, but the 3 blade has more catch for lack of terminology on my part. So if you go against currents etc, a 3 will not slow down as much as a two blade. BUT< the 2 blade has less drag sailing. Hence a folder if you go with a 3 blade would be the best option for your boat. Good motoring, along with low drag sailing.
She drives me boat,
I drives me dinghy!