You can change the font size in the IDE terminal and the text editor by using
CMD + +/- on OSX and
ALT + up/down on Windows.
CMD/CTRL-0 will also reset the size to whatever is Atom's default (which is configurable).
With the latest version of the IDE, you can also use custom fonts to suit your coding style. Just go to Learn IDE > Preferences > Packages > Learn IDE> Settings and choose your font!