Not sure about 6'6, but fyi I recently asked the same question to True Ames re: a 7' Liddle M3P and got this response:
"I'd opt for the Powerglide in a 7' Liddle. The additional rake on the PG is going to allow it to feel more fluid when your foot is not over the fin. I would recommend going with the 8.25 PG because it will work in the 7.0 and also have versatility if you want to throw it in another board."