You may or may not want to experiment with different prop sizes, just to see how the engine and airplane are affected, but I wouldn't let altitude be the primary factor in the decision.

My brother took his Astro Hog from Greenville Texas (541 feet ASL) to Thayne Wyoming (5993 feet ASL) and was concerned about flying at altitude. He runs a .91 four stroke with a 14x6 prop. He said the plane flew the same in WY as it does in TX and the carburetor required very little adjustment.

Carrell