CCW_tester,
Rats! Can't find my ruler. So, with the limits of my not so knowledgeable knowledge of metrics kept in mind, I think that 75mm might be sort of large.
One way of looking at it is by how much water a particular prop will move in one revolution. If the pitch of the prop is about the same for each, the less number of blades the prop has the less water it will move at the same rpm as a larger two bladed prop. If you increase the number of blades and they have the same pitch as that two bladed one, it's gonna move more water per revolution. A two bladed prop will move as much water as a four bladed prop if it spins faster (all else being equal, prop diameter and pitch). That's fine till you reach the point where the prop starts to cavitate, or the motor hasn't got any more 'speed' to it, or it over heats, then you have a drastic loss of speed. So, you might think of the number of blades per prop as a substitute for motor speed, sort of. The more area a prop has the more torgue/power it takes to spin the thing, more resistance or a bigger load on the motor. Low torque, high speed motors don't do as well with props with a lot of blades. The same sort of thingy applies to the prop's diameter too, to some extent.

***This is a generalization so don't expect it to be very 'accurate'/scientific/true! It does give you a vague idea of what's happening, sort of. ***

All this can be figured out mathematically. Uh, don't look at me, I'm too lazy and don't know how anyway. How would I do it? I'd use the prop that came with the boat, and then go from there (hope one comes with the boat!). Lot'a help, huh?
- 'Doc

(A little 'too much' prop is probably 'better' than 'not enough'. There are limits to that too! Don't ask! I don't know that either.)