"1 ilm: An adult Hyuran thumb, a ripe rolanberry."

Interesting bit about this one - in Eorzea it currently has a standardized implementation rather than just being an approximation (like it might be among a particular community). Details vary based on who does the telling, but basically a pirate/sailor started a fight over believing he was given shorter cloth than what he paid for and the Admiral at the time cut off his thumb, had casts in metal of it made, and then had those sent out as a shared standard for those engaged in trading.