Actually, Dana is correct.
A propeller blade operates in a perpetual downwash of the preceding blade, just like the airplane tail is in the downwash of the wing in front.
Note that there is a difference between downwash and wake.
Downwash is simply a changing of the direction of flow. If you visualize the airflow over a wing, the air ahead of the wing takes an upward turn in "anticipation" of meeting the wing---and then aft of the wing it turns downward, in reaction to the lift produced by the wing. So first comes upwash ahead, then downwash behind and the transition point is right about the quarter chord point, which is the aerodynamic center of an airfoil (generally).
Wake happens only when an airfoil is stalled. It is simply very turbulent air that does not flow in streamlines, but is churning all over, behind the stalled airfoil (or any unstreamlined object; a cube in a flow of air will produce a large wake for example). The problem is that a wing or prop blade flying through such a wake will experience much higher drag.
This is not true of flying through a downwash, although there is a small aerodynamic penalty here too. The wing or prop blade flying in downwash will need a higher angle of attack to make the same lift. Think of it as flying "uphill," because the airstream is coming down at an angle that the wing must "climb." The extra alpha (angle of attack) needed means the lift vector will be tilted back a bit, which means an additional drag component.
So a prop blade will always be in downwash, but it will only be in a wake if the prop is stalled. Stalling of props usually is only seen with fixed pitch props with high pitch, which would be used on an airplane with a high top speed, say 200 mph or so. In that case, the blade can stall when the airplane forward speed is slow or standing still---because of the high angle that the blades are set. That is why fast planes need in-flight adjustable props, so their low-speed performance does not suffer.
As for the question of 2 blade or 3, the two blade will generally perform better, partly due to less downwash as Dana said. But there is more to it, such as as blade diameter, etc.
Now about the specific question, I'm assuming the engine is about 45 hp so a good prop should be making about 250 lb of thrust. A good diameter would be about 66 inches, for an all round prop. This would be for a plane designed for about 75 mph top speed.