If it is a scale model, and you wish to remain as authentic as possible the size and style (blade count, shape etc.) of prop is dictated to you already by the 1:1 version. In the case of very secret modern designs we have to give our best guess e.g. modern propulsors like on Astute and Vanguard.
Assuming we don't have to follow a prototype, then generally speaking a 3-blade prop is usually the best choice for efficiency, with perhaps a two blade for very high speed subs. Larger props are more efficient than small ones, within reason- very large props create a large amount of drag, so there is always a trade off.
With regards to pitch, best efficiency is generally to be found from about 0.8-1.5 times the diameter of the prop, with most props designed for scale electric models tending to be around the area of 1-1.2 pitch.
It would appear that a coarse pitch prop is the best choice, bearing in mind that it will generate more thrust per revolution assuming the same diameter thus allowing a slow revving motor. This is kinder to shaft seals but comes at the expense of generating a larger torque roll effect on the boat, which can be an issue on single screw boats if it happens to have a modest metacentric height for agility. Also coarser pitch props tend to slip more.
In practice, from my own experience and observations of other craft, a pitch ratio of 0.8-1 seems to be about right for model submarines with single screws. Boats with contra rotating screws don't have the issue of torque roll to deal with, so you are free to choose the pitch of your choice.