This might help you choosing the right size
Btw, I used the Decade SL (11/12 model) and it's a great boot, just keep in mind two things:
1) the inner shole (inlay?) is thermoformable, so in few days on the monuntain it will be roomier(i think you can gain about 5 or 10 millimeters, so choose proper size!
2) the boot flex will get softer after 30-40 days on the hill (depending on your weight and your riding style), than it will be in the softy-freestyle section for the rest of his life!