emacs-font.el

(defvar my-emacs--font-size 12)

(defun my-emacs-font ()
  (format "-*-Ubuntu Mono-normal-normal-normal-*-%d-*-*-*-m-0-iso10646-1" my-emacs--font-size))

(add-to-list 'default-frame-alist `(font . ,(my-emacs-font)))

(defun font-bigger (size)
  (interactive "p")
  (if (eql size 1) (setq size (+ my-emacs--font-size 2)))
  (setq my-emacs--font-size size)
  (setq default-frame-alist (cons `(font . ,(my-emacs-font))
                                  (assq-delete-all 'font default-frame-alist)))
  (set-frame-font (my-emacs-font) nil t))