Help with Daft Punk Helmet scaling

I started on making my Thomas daft punk helmet from a pep file that i got and as soon as i finished the visor i could tell that it wouldnt fit me when i finished it. Idk how to scale the pep file that i have or can someone help with that? I wear a 7 5/8s hat size or what kind of measurments would someone need?

 

This is a tutorial that i used for my helmets. For my first 2 builds they are wearable.I may need to cut a little off on the underside of the helmet but other than that they fit great.

http://www.cosplay.com/showthread.php?t=191118

I believe i used Method 1.

Good luck and hope it helps.