Brian,

Most "normal" muffler equipped, .60 class engines make their maximal horsepower at around 13,000 RPM. Please see the .60s shootout that was aired in MAN of May, 2003, by David Gierke.

This means that at maximum level speed they must spin around this RPM. Translated to static ground RPM, this would mean propping them to spin around 11,500 RPM.

Using a large prop that will spin more slowly on the ground, will never allow the engine to utilize its maximum available horsepower output (Unless it goes into a very steep dive). Somewhat like using a smaller engine.

Using a smaller prop, that will spin faster on the ground, will cause the engine to spin too fast, again producing less horsepower than it can.

For most normal .60, an 11x8, or a 12x6 are closest to this target.
With the 11x7 almost any .60 is under-propped.

Stronger .60s, that attain their maximum HP ratings at different rpm levels (don't believe the engine manufacturers...) can be propped differently.