helmet size help?

[merlowe]

hi everyone! i just recently built a pepakura ironman helmet and was pretty disapppointed when i saw that it was to big.
i am still really new to sizing and pepakura designer and was wondering if there is any formula or certain measurements i need to take to get accurate sizes, i ask this because i plan on building a full ironman suit, i just dont know 1)what place to measure, 2)were to input this gathered measurements.
ie: the mark IV chest piece on ironman, would depth be from the side of my rib cage to my chest? thanks in advance, and im sorry for being a noob..
im sure you guys deal with questions like this all the time. :$
a pic of my failure:

 

[PepMaster]

If you are using the Robo3687 Pep Files for the Suit they should already be scaled and work well for about a 6 foot tall person, as for the helmet, 260mm high is what I have seen used for some of the helmets which fits well but this helmet does'nt look too oversized from what I see.

 

[merlowe]

im 5'8" (short)lol
its just that, ive seen other peoples helmets and they seem way more form fitting and sleek.

how should i measure my head to obtain proper height width and depth?
cheers!

 

[PepMaster]

From chin to crown add about a inch convert it over to millmeters and then in pepakura designer, change the scale and put whatever you got in mm for height into the height box.