Well of course you have to be able to fit your head inside the helmet, but the main sizing issue to really tackle is how the helmet looks in proportion to your body while you are in armor. If you are a big guy you will need to go bigger than a 1:1 Boba Fett template. If you are a smaller guy you might have to downsize. The size of peoples melons isn't always relational to their body size, so some guys have tons of room in a 1:1 helmet and it looks correct, while others may have nose prints on the visor.....but it still looks correct for the costume.