Yes I think about it since long. I was about 10-11 years old when I think of a correspondance.
I always considered a 1x6 plate as a 6 feet rule. 1x6 plate is a bit more taller than minifigs. Peoples are a bit shorter than 6 feet habitually. It correspond well. It make's minifigs to be about 5'5" I think. About the mean tall of true people. Plus ! mean Middle ages peoples were a bit shorter than today's people.
So it's easy to remember. 1 stud = 1 feet.
6 stud = 5 bricks tall. So 5 bricks tall = 6 feet.
For the correspondance in meters it's easy. 6" is about 2m.
1 stud = 1/3m, 3 stud = 1m.