biologically, it is based on the 9.(571428) ratio
compare persons height with either penis, face width, upper leg depth, hand length, face height, 1/2 shoulder width, tummy depth and foot length wo toes.
you will see how this normalises the person according to height and is based on my personal dimentions since I was considered having a stardard body type and are used to fitting into off-the-shelf cloths before they all came from china.
Use blenders 3D ruler and ensure they are at least within a few percent.
for example, 6′ would equate to 7.5 inches.