@neauoire Btw, if you want to work with real units, we've learned these approximations in school:

1cm is the height of a finger
1dm (=10cm) is the width of your hand
1m (=10dm) is the distance from your fingers to the other shoulder (useful for measuring ropes/garn) or the size of a moderatly big step (useful for measuring distance).

1mm is easy to guess, but maybe the thickness of your shirt?