gistfile1.txt

(defun dotspacemacs/increase-font-size ()
  (interactive)
  (set-face-attribute 'default nil :height
                      (ceiling (* 1.10
                                  (face-attribute 'default :height)))))

(defun dotspacemacs/decrease-font-size ()
  (interactive)
  (set-face-attribute 'default nil :height
                      (floor (* 0.9
                                (face-attribute 'default :height)))))
                                
;; Later, in your dotspacemacs/config...

(evil-leader/set-key "of+" 'dotspacemacs/increase-font-size)
(evil-leader/set-key "of-" 'dotspacemacs/decrease-font-size))

;; Then you can SPC-o-f-+ to increase font size globally or SPC-o-f-- to decrease