IdleX provides this with the ZoomFont.py extension. (See http://idlex.sourceforge.net/extensions.html#Misc) It is useful for showing code on a projector so that students can easily read the screen.

The implementation in ZoomFont.py does not change the font size found in IDLE's configuration. Instead it queries the font size in the text widget and scales accordingly with a relative offset. 

IDLE needs a better way to broadcast font configuration changes. Right now, the Code Context extension uses polling every second to check for font changes. I bring this point up because properly incorporating font zooming may require refactoring some code. Incorporating Line Numbers, from #17535 would likely need to rely on this font size broadcasting as well.