Minimum feature size for labeling - custom/override map units

[wayfaringrob]

It would be convenient, for example, to label rivers that are a mile or longer, but this usually isn't easy unless your map units happen to be miles. If your map units are feet, that's not difficult math, but if I wanted to do anything other than a mile or my page units aren't feet, I'm having to get out a calculator.

Allowing for a custom map unit here would be convenient in cases like this.

(It would also be super handy if it told you what your map units were right there, especially when you're working in layout view.)