How to change the default font size of XTerm?

If you don't already have one, create a file in your home directory named .Xresources to store your preferences for various X programs. Append a line to the file such as

xterm*font:     *-fixed-*-*-*-18-*

This informs xterm to use the 'fixed' font at size 18.

From here, you can either restart X or run xrdb -merge ~/.Xresources in a terminal to incorporate the changes you've made. All new xterms should now have the font change.

If you run man xterm and go down to the RESOURCES section, you can find a wealth of additional, configurable xterm options.

Use -fa and -fs parameters:

xterm -fa 'Monospace' -fs 14

You can also Ctrl-Right mouse click for temporary change of font size