Aterm only supports bitmap fonts, and I don't think there are that many included. You can try using something like
"-adobe-helvetica-medium-r-*-*-18-*-*-*-*-*-*-*" as the font name in .Xdefaults.
Alternatively, use a terminal emulator that supports Truetype fonts, like Terminal or lxterminal. The font size should be easily available in their preferences.