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 font2Search for font2