From a purely layman's perspective, why couldn't you get a head to toe photo of a subject, then have the subject enter in their height, then use that measurement to get the relative measurements of other body parts? You'd need a way to quickly identify the points of the body, but i can't imagine that is the difficult part here? The head to toe photo of the subject would need to be wearing skin tight apparel, such as underwear to ensure clothing doesn't throw the relative measurements off.

Interested to know where my assumptions here don't line up with reality.