I have a tiny laptop: 8.9″ diagonally. With a 1024×768 pixels screen
resolution, things can get *pretty* small. The following functions use
the gnome-terminal-style shortcuts (Ctrl-plus, Ctrl-minus) to change
the font size without the mouse:

(defun sacha/increase-font-size ()
  (interactive)
  (set-face-attribute 'default
                      nil
                      :height
                      (ceiling (* 1.10
                                  (face-attribute 'default :height)))))
(defun sacha/decrease-font-size ()
  (interactive)
  (set-face-attribute 'default
                      nil
                      :height
                      (floor (* 0.9
                                  (face-attribute 'default :height)))))
(global-set-key (kbd "C-+") 'sacha/increase-font-size)
(global-set-key (kbd "C--") 'sacha/decrease-font-size)