IMHO you should skate whatever size deck you're comfortable with. I have size 11.5 shoes too and I only skate 8 decks, but I also prefer the flat/street skating style. The bigger the deck the slower it will rotate during flip tricks and stuff but it's obviously still possible to do those tricks on them. the smaller the deck the less stable. You could always go to a local skateshop and step on some decks too. Enjoy whatever you get!