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.
You might try to print the inner parts and the dome base with frame on 1:1 scale then assemble those parts using just plain paper to see if it fits you. If not, you can do a resize of the templates with any percentage within the printing menu.
Assembled should look like this:
Ya thats what mine looked like, just smaller, ill try and print them at a bigger percentage.
We really wouldn't quite know the proper proportion though until we have armor constructed, would we? Cranktime and I are building together since we live a block away from one another. How will we know w/o armor?