One way is to purchase the full software, which will allow you to scale it to differnt proportions. As far as myself, I have been very lucky, only having to get the free version and finding files that fit me (but then again, im on the younger side, and my head isn't real big).
Keep in mind that this particular helmet is in three pieces (the back peice, chin, and faceplate). The seams are made clear in the pepakura file I used, so If you would like me to send you it I'll send you a mesage with the link.
Glad to here you have an interest in pepakura, its a rewarding and fun hobby! Good Luck, Freelancer521