From the Emacs help
C-x C— runs the command text-scale-adjust, which is an interactive autoloaded
It is bound to C-x C-+, C-x C—, C-x C-=, C-x C-0.
(text-scale-adjust &optional INC)
Increase or decrease the height of the default face in the current buffer.
The actual adjustment made depends on the final component of the
key-binding used to invoke the command, with all modifiers removed:
+, = Increase the default face height by one step
- Decrease the default face height by one step
0 Reset the default face height to the global default
C-x C— is really, really close to C-x C-s on a Dvorak keyboard. If ever you hit it by accident, hit C-x C-0 to get your Emacs looking right.