Prop sizes are usually recommended sizes for a given engine and are to be taken with a grain of salt for the application. You may want something with more pull but less speed. Then something with less pitch than a 'recommended' prop may be your choice. Or, more top end speed, with less pulling power.

Think of a prop as a transmission in a car. The lower gears will give you more pullint power, but much less speed for a given RPM. Then, the higher in gears you go, the more speed for a given engine RPM but less pulling power.

Pick a prop in the recommended range, try it out. If it does what you want, stick with it. If not, and you want more pull... lower pitch. More speed... higher pitch. All of this for a given diameter prop, of course. Things will change as you change the diameter of the prop, but with the general same results.

Bottom line, experiment and pick what performance you want and go with that.

CGr