I have an APC 14-7 on my Goldberg Skylark 70 with the OS .75 AX engine. It flys just fine, suites my needs. I've really never needed to experiment because it just worked and I left it that way.. if it ain't broke, why fix it!! All the recommended props are fine.

The chart put up by spacemonkey is a tad light for this engine, which performs more like a 90 than a 75. Prop it accordingly.

But, as mentioned, it would never hurt to have a few various sizes around and try them out. See what one(s) get you off the ground the best, do good vertical maneuvers, and slows the plane down appropriately for landing. There will be one that stands out for your particular application. Buy a couple of them, then enjoy your flying!

CGr.