RE: Maximum altitude??
Thrust determined by newtons law (throw air in one direction, and you get pushed by the same force in the other direction), is determined by M * V.
That is, the mass of the air the propeller throws, multiplied by the velocity it accelerates the air with.
This means, the same amount of thrust can be produced by take a lot of air and give it a small acceleration (big prop, spinning slowly).
Or by taking a small amount of air and give it a big acceleration (small prop, spinning quickly).
However, the energy needed to produce this thrust is determined by 0.5*M*V².
That is, doubling the mass accelerated will double the energy input needed.
But doubling the velocity of the same air, will quadruple the energy needed.
Thats why big props spinning slowly is more efficient.
But big props spinning slowly is also heavier, so size has to be kept within the limits of reason.