So one concern is that we're introducing non-standard font-sizes -- unless this ends up being a scaling factor based on ems (which is a whole 'nother kettle of fish in terms of code usability). 

I would argue for trying to rigidly keep to a few sets of font sizes right now for maintainability and/or boosting $FS_small rather than creating a plausibly idiosyncratic size.

Matching the helper text (sorry, can't recall proper class name) sizing seems like a reasonable strategy, assuming it's also $FS_small (I think it is, IIRC).