Emacs lisp interactive function to change font size in GUI easily.

change-font-size.el

(defun change-font-size (size)
  "Change font size (clearly `:height` in `face-attribute`) to given SIZE."

  (interactive
   (list
    (read-number
     (format "Input font size (current=%d): " (face-attribute 'default :height))
     nil)))
 
  (set-face-attribute 'default nil :height size)
)