The load a prop puts on an engine is actually determined with a lengthy calculation... but you find a prop that works well, then if its a .40 size (or close) you can trade 1 inch pitch for in inch dia within the 9 to 12 inch dia range and get decent results.

The primary rules of thumb for for props tend to center on appx .40 ci engines. Power has a cubic factor... prop blade areas have squared factors with dia change (blade "span"). It just happend that they tend to match up well in the range where a .40 engine is used.