I'm using 48' for my LittleAxe with Norvel .061 and 6x2 prop, or 5x3 3-blade.
Generally use 42' for most everything else, combat and sport planes; 35' for my weaker .049s (Cox, Brodak I, etc.).
15' for my .020 1/4As, because that's all the room I have in my backyard! I think they would handle up to the old 27' 1/2A standard length of yore.
Per stock engineering answer: "It depends."
--Ray