If the guy who works the hobby shop is worth anything then he should be able to tell you... If not then you need to post what motor/battery/esc you are using. If you are using an 11x4.5 then you might be able to go to an 11x4, 11x5, 12X4, or 10X5 with no problems... but then again you might just fry something. It also depends on what brand prop you have available. Is it the thin orange ones(GWS) Or slightly thicker grey ones (APC) or ?? I can assume the s in the prop rating you gave is a slowfly prop, does it start out thin near the hub then get really fat and looks pretty rounded or is it pretty uniform and relativly straight?