osx-change-font-size.el

(defvar *default-font-size* 13)
(defvar *default-font-name* "Menlo")

(defun increase-default-font-size ()
  "Increase font size"
  (interactive)
  (incf *default-font-size*))

(defun decrease-default-font-size ()
  "Decrease default font size"
  (interactive)
  (decf *default-font-size*))

(defun compose-default-font ()
  (format "%s-%d" *default-font-name* *default-font-size*))

(defun set-font-as-default ()
  "Set the (internally registered) font as default"
  (interactive)
  (set-face-font 'default (compose-default-font)))

(defun increase-and-set-default ()
  "Increase font size and make the increased font default"
  (interactive)
  (increase-default-font-size)
  (set-font-as-default))

(defun decrease-and-set-default ()
  "Decrease font size and make the decreased font default"
  (interactive)
  (decrease-default-font-size)
  (set-font-as-default))

(global-set-key (kbd "M-+") 'increase-and-set-default)
(global-set-key (kbd "M-_") 'decrease-and-set-default)