how can we measure "yalm"?

[Velox]

It kinda does use real world measurements.

Yalm = yard
Malm = mile
Fulm = foot
Ilm = inch
Ponze = pound
Onze = ounce

This^

Although to many it probably seems odd that the American units would be used as a starting base for a Japanese game, but it does make some sense due to the commonness of the units outside the U.S. and how using metrics would seem bland given they are already used everywhere in the real world. It helps give Eorzea more flavor.