Propeller selection is its own deep rabbit hole, but a few fundamentals apply broadly across most gas and electric sport and scale setups.
Diameter and pitch are doing different jobs. Diameter is broadly related to how much air the prop can move (and therefore how much static thrust it can generate), while pitch relates to how far the prop would theoretically travel through the air in one revolution, which affects top speed and how the engine or motor loads up at different RPM. Manufacturer-recommended combinations for a given engine or motor are a sensible starting point rather than something to second-guess without good reason.
Gas engines generally want a specific pitch range for the engine to run happily, and going too aggressive on pitch can load the engine down below its efficient RPM range, hurting both performance and engine longevity — this is one area where following the engine manufacturer’s recommended prop range genuinely matters, rather than being an overly cautious suggestion.
Electric setups have more flexibility but more room to get it wrong, since motor, ESC, battery, and prop all need to work together within safe current draw limits — a prop that’s fine on one motor/battery combination can be genuinely unsafe (overheating the motor or ESC) on a different combination, even at the same nominal size.
Balance and true running matter as much as size selection. An out-of-balance or slightly warped prop introduces vibration that affects everything downstream — engine mounts, fasteners (see Day 17), even radio gear reliability over time — regardless of how correctly sized the prop otherwise is.
When in doubt, start conservative and work up, particularly on a new engine or motor combination you haven’t run before — it’s easier to notice a prop that’s slightly too small than to recover from over-stressing a new power system with too aggressive a first choice.
You can browse our Carbon Fiber Propeller For RC Fixed Wing Airplane 19×8 19×10 22×10 23×8 23×10 24×8 24×10 Propeller to see this in practice.