For hover applications where the air doesn't need to be moved fast, maximum rotor diameter is always the best, because it lets you move the blades relatively slowly, avoiding parasitic drag (air friction) while still moving a lot of air. That's why helicopters have such big rotors. With this hoverbike, practical matters dictate that the prop diameter is very limited, meaning that in order to move enough air, you need to spin the blades really fast. In order to not waste all energy on air friction, the blades need to be very slim and low-drag.
Think of the wings of a jumbo-jet. At low speeds for landing, slats and flaps are extended to make a big curved wing. It creates more lift, but it's inefficient. It wastes much more of the kinetic and potential energy of the plane, slowing it down. At cruise speeds the slats and flaps are retracted, making the wing very streamlined and giving it maximum efficiency.
Finally, there's the pitch or steepness of the propeller. The propeller has the lowest drag when cutting the air at zero angle. Of course at that point it doesn't generate any lift either, so the efficiency sucks. When the angle is increased, the prop starts pushing air and creating an equivalent induced drag. The propeller is still almost parallel to the air, so the parasitic drag remains small. Since most of the drag is induced drag at that point, the efficiency is high. If the pitch is increased further, the propeller cross-section against the air increases and parasitic drag goes higher. Sure, it pushes air faster when spinning at the same speed, but since the parasitic drag is high, it can't be spun as fast anymore. High-pitch propellers are used in fast planes, because when the planes move quickly through stationary air, the effective angle of attack of the propeller (the angle at which the forward-moving prop meets the non-moving air) decreases, making it efficient again! In hover applications the prop doesn't move forward through the air at anywhere near those speeds so it's most efficient to use a low angle that provides the optimum lift and minimum drag.
