Well, it doesn't change size based on the wielder. It changes size based on its will and what it percieves will bring it the result it desires. Which is admittedly way more complex and abstract. This is why I said knowing more about it's properties would be extremely important to figuring out how to use it.

If it only changes size in one direction, like growing bigger to slip off a finger, but the band width never changes, then it would still be extremely useful for some measurements. We also don't know if there's a maximum or minimum to it's size changes which would greatly matter.