The propeller on your aircraft is actually a set of wings, designed as rotating airfoils. The selection of an appropriate propeller for your experimental plane will require that you do some research or follow the specific advice of the aircraft designer. The primary choices are fixed pitch or constant speed. Constant speed props can be adjusted during flight to be more efficient during climb or cruise. Constant speed props are more popular with high-performance (and higher-priced) aircraft designs.A fixed pitch propeller cannot be adjusted during flight, though some can be adjusted on the ground prior to flight. You should choose a propeller that’s designed for the best performance you can get from that engine. Your best choice is to install the propeller that comes with the kit or is recommended by the plan manufacturer. Don’t try to be creative.
Two blades or three? Leave that, too, up to the aircraft designer and make sure you buy and install whatever the plane manufacturer recommends. Experienced builders of experimental aircraft have been known to design and fly custom props, but it typically takes some engineering and woodworking skills to make it work safely.
One more point: Many smaller aircraft engines operate at a higher speed (revolutions per minute or RPM) than is most efficient for the propeller to turn. These engines use a reduction unit that delivers the correct power, in RPMs, to the prop. Make sure you know if your aircraft engine needs one, which one, and how to install it.
"Man cannot discover new oceans unless he has the courage to lose sight of the shore."
-- Andre Gide