Adjusting interactively the font size is Emacs is pretty easy. Three keybindings refer to the mighty text-scale-adjust command:

  • C-x C-+ (increase font size)
  • C-x C-- (decrease font size)
  • C-x C-0 (reset font size to default)

After using any of the keybindings you can proceed to adjust the font size further just by pressing +, - or 0 respectively. text-scale-adjust is mostly a wrapper around text-scale-increase and text-scale-decrease.

You might want to bind them C-+ and C-- (generally a bad idea since you won’t be able to type stuff like C-- some-command anymore) to get more common keybindings for the operations.

(global-set-key (kbd "C-+") #'text-scale-increase)
(global-set-key (kbd "C--") #'text-scale-decrease)