7.5 is a generally accepted, average norm... I learnt that at art college. Not sure exactly how scientific it is... or if it is just a "rule of thumb". The latter I think... quite literally, i.e. when doing a drawing you'd use your thumb, held out in front of your eye, to get the head size and them move it down in increments...

But I did a lot of life figure drawing... and anatomy drawing... at art college, and every actual human model was different and deviated from that to varying degrees... but at the same time it was always approximately right.

I guess if you are trying to make a geometric model that can be morphed, or whatever, to represent the broadest spectrum of humanity, then following that 7.5 heads principle is probably a relatively sound thing to do...

;-)