Several pay-ware and free-ware programs out there will approximate thrust, rpm etc. Here is a thrust calculator to get you started...

http://www.bmaps.net/

Look under "Goodies"

Theoretically you should see several hundreds rpm increase moving to a smaller diameter but keep in mind that that other factors like prop brand, fuel content (more nitro CAN increase power/rpm), ambient temperature, barometric pressure, needle setting and more all will affect engine performance and thus prop speed. It may vary from day to day. There is just experience and 'rule-of-thumb' to guide your prop selection. Try several different props to see what fits you and your plane best.

Here's a chart I use to get me started on prop selection.