My method only applies to the FS HD helmet but here's what I did when I worked on DF4L's model: I first measured the distance between my ears. I then assembled the part of the helmet that are directly adjacent to the helmet opening. I then measured the width of the helmet opening and then re-scaled the model until this width equaled the distance between my ears.
I know it's not efficient but with a little patience, you can get the right scale for your helmet.