If you don't intend to use the [Aa] menu all that much, then I suggest editing the reader.pref file in the system folder (or use the Calibre plugin), I *think* that still works to set the font size.
The only other choice is to rescale the font with a font editor, which may not be as easy as it sounds.
Sans Serif *is* the regular font
. The hack lets you switch three font families out of the four (basically, everything except condensed).