What helmet file are you using? I know Shark's helmet has an original scale of 1.00 and it's supposed to fit your typical 6' tall person. I don't know what you know about scaling, but since we know this, the formula is easy.
First, divide your height by the original intended height. In my case, it would be:
69 / 72 = .958333
Now take the number you get from this, and multiply it by the scale in Pepakura. Since the scale for this helmet is 1.00, you don't have to do anything, but for the sake of demonstration, do this:
.958333 * 1.00 = .958333
You can add a bit or take away a bit depending on your head size, but this should get you in the ballpark. I hope this helps.