The magnum website suggests suitable props for their engines.

Here's the link to the stats page. http://www.globalhobby.com/magnum/magxl.htm

Looks like a 9x5 or 9x6

Often, if you try out a couple of different props in the recommended size, you will find one that works measurably better than the others. It's a heck of a lot easier than trying to hopup the engine.