my hub is 41mm in diameter and is a pretty good fit for 22" props, you could go a bit larger.

yes, M5 screws are pretty much the standard. grade 12.9 is the best way to go and even those can get sheared if not kept tightened properly. the 34mm pattern with M6 screws would be much more robust but might be too big for the prop you go with....you could always do both patterns in the hub and decide which to use later.