I use it on windows for the font size.
edit: I'm surprised you are getting a control character for it.
edit: Ah. I think you are using control key + underscore. That might actually be the way to type the control code you are seeing represented by printable "US"
edit: what keyboard localization are you using? My keyboard has underscore and minus on the same key and I have to use control+shift+underscore (not minus) to decrease the font size. The plus key is next to it, and control+shift+plus increases the size. It looks like the FF web browser doesn't care if I press shift or not.
Craige