You just have to buy a selection and try them.

Going down in pitch with out changing diameter will usually slow the plane down and increase torque as above, but sometimes the plane will go faster and have more climb at the same time. The reason for this is that the engine will be less loaded and will pick up RPM, sometimes enough RPM to make up for the pitch difference. Weither this will happen or not is not really predictable, you just have to try it.

Also, be sure to try multiple brands of props. An APC and a MAS in a 10x6 will pull the same plane very differently, for example.

You can also frequently go a bit bigger or smaller than the manual states.

If you have a tach, that can help you keep from putting on too small or too large a prop. For a .61, I'd say you want the top RPM on the ground no higher than 16k, and if you turning any less than 10k the prop is too big. Any prop that puts the engine in that range will be "safe". Then you test as mentioned above and pick what you like best.