If you are still using command-line switches to setup your terminals, I recommend you have a quick read about .Xdefaults or xrdb, you can start with xrdb(1)'s man page. This is how you set specific application resources on old-school X programs, such as xterm(1). You can usually find all the resources for an app on its man page.
If you have huge monitors and got used to zoom web pages with Ctrl+MouseWheel and Ctrl-+/- then this is for you, assuming your have terminals with multiple font sizes defined (see next section).
The default keyboard bindings for increasing and decreasing the font size are Shift+Keypad_+ and Shift+Keypad_-. This is all fine and good if you have a full keyboard but gets annoying if you have a laptop and need to press 6 keys to get a virtual numeric keypad, or use the Ctrl-RightClick menu:
*VT100.Translations: #override Ctrl <Btn4Down>: larger-vt-font() \n\ Ctrl <Btn5Down>: smaller-vt-font() \n\ Shift Ctrl <Key>=: larger-vt-font() \n\ Shift Ctrl <Key>-: smaller-vt-font()
The above is useful of you can define your own font and sizes. By the way, if you haven't tried Inconsolata in your terminals, give it a shot, it is worth it:
XTerm*faceName: Inconsolata XTerm*faceSize: 12 XTerm*faceSize1: 8 XTerm*faceSize2: 9 XTerm*faceSize3: 10 XTerm*faceSize4: 12 XTerm*faceSize5: 16 XTerm*faceSize6: 24
How often do you have a terminal opened somewhere in your files and need to open a new terminal at the same exact place? Of course, you could use xterm &, but what if you are within vim, within mutt or just can't run any shell command?
XTerm has a cool function that is not bound to any key by default, I personally bound it to F12, here is the code, of course, remove the '#override' if you plan on mixing it with the above snippet:
*VT100.Translations: #override <Key>F12: spawn-new-terminal()