Hah! I am usually 13.5 or 14 in shoes. Snowboard boots are all over the place. Some 13's fit bigger than other 14's. You know the story. 15 is really tough in that most manufacturers don't even produce them.
Please measure your foot using this method:
Kick your heel (barefoot please, no socks) back against a wall. Mark the floor exactly at the tip of your toe (the one that sticks out furthest - which toe this is will vary by rider). Measure from the mark on the floor to the wall. That is your foot length and is the only measurement that you will want to use. Measure in centimeters if possible, but if not, take inches and multiply by 2.54 (example: an 11.25 inch foot x 2.54 = 28.57 centimeters).