I think people normally use 26x10 props on the 70 single, so your 80 should be able to use the same size prop.
My Zenoah GT-80 spins a MenzS 24x10, but since the King is designed with low rpm "grunt" in mind, unlike the Zenoah which likes high rpm, I think you should be able to stick with the bigger 26" props, thereby getting a very silent setup.