Well are you wearing an american size 13? You sure you are measuring your foot properly?
Gnu has a normal and wide version for the carbon credit. if your foot is actually 27cm, that is a size 9, and you will be fine... as long as you boots are actually a size 9. You want to have a bit of overhang on your board, since your foot is elevated, you just dont want so much so that your feet drag when you carve.