Interactive Font Size Adjustment in Emacs. About time.

(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)