its not a myth - but its not foolproof either.
there will always be exceptions.
other apparent indicators are hand size, nose and ear size, though i think the nose and ears are less reliable - especially since the ears continue to grow throughout your life - that would mean all old men are ....mmmm ...
the size of your foot is the same as the length of your forearm, from wrist to inner elbow,
your nose is the length of your thumb,
your hand span in the length if your face,
your arm span is your height,
the top of your ear lines up with the outer corner of your eye
- there are other things that correlate so there are medical and scientific reasons for these things to match up.