Set the options for font1, font2, ... font6 (classname Font1, ... Font6) to correspond with unreadable, tiny, small, medium, large, and huge in the .Xresources, e.g.

 xterm*font2: -*-terminus-*-*-*-*-12-*-*-*-*-*-*-*

This is in the manual [Search for font2][1] 


 [1]: http://invisible-island.net/scripts/sample-output/vh-man2html-xterm.html