This topic has been covered alot so do a search and youll find tons of info.
Basically it works like so the higher the diameter the more thrust, the higher the pitch the more speed. Usually when choosing a prop for an engine you can plus 1 and minus the other. exp if you have a .40 engine you can either have 10-6 11-5 or a 9-7. However having to big or small a prop can be harmful to an engine. Theres tons of charts that show which prop best fits which engine. Hope this helps.