I'm guessing you're using the Dung0beetle EP.II helmet? Scaling peps is the hardest thing. Their biggest strength, that they a based entirely on a digital file, is their biggest weakness in this respect since they have never been test fitted... I've built a lot of peps that don't fit. In my build project of one of Ekip's EP.II helmets I'm on my third full sized helmet due to fit issues (and I've cut out three others that simply got scrapped during the first build session!).
Here are my Tips I'm also guessing you're using pepakura designer:
1. Pick your scale - 300mm is a good starting point (I believe this comes from Stormtrooper helmet land) but remember to take into account any extras that add to the height and not the helmet size (fins, Mohawks, rangfinders etc.)
2. Print out the helmet on normal paper (preferably recycled - you'll see why soon). But Don't cut out all the pieces straight away (you're probably past this point already)
3. Find the pieces that make up the "brow" or "Hatband" portion of the helmet (the parts that fit around your head at the height of your eyebrows) and cut out only these parts and join them together to form a ring for test fitting make sure you also allow for padding - I'd want at leat 10mm (3/8 inch) gap all around and probably more like 20mm.
4. Repeat steps 1 to 3 until you get a good fit. Hence the recycled paper at step 2.
5. Print out the helmet on whatever paper or card stock you want to use for the "final".
6. Cut-out all the parts and start building...
I wouldn't worry about things like neck rings not fitting, there are ways around this once the helmet is built. The main thing is having a helmet that fits, overall.
Good Luck