This is useful for two stroke prop selection:

http://www.top-flite.com/accys/topq5000a.html

The chart suggests something between an 11x8 and 13x8 for a 0.75 ci engine.

Normally, one would lightly load the engine during break in. So if the manual suggests a 12x8 for break in, then a 13x8 or equivalent might be about right after break in. I see on the Super Tigre's web site they suggest a 12x6 or 13x6.

To guesstimate prop "equivalence" just add the diameter and pitch to get a single number that represents the prop's load. A 13x8 is 21. So, a 14x7 would be about the same, as would a 12x9. For a more refined analysis, there is a program floating around the net called "ThrustHP" that is interesting.

Generally, one tries to match the prop pitch to the speed of the plane so that the prop operates most effeciently. Slow flying planes use low pitch props, high speed planes use high pitch props. The engines speed (rpm) factors into the equation as well. There is no hard rule, however. It is a matter of preference to a certain extent. You may have to try a couple to see what you like. Once you get used to the plane, you will notice the difference between different props and will likely have no problem identifying which one "feels" best.