This package watches the current display configuration (name, pixel width and height, physical dimensions, and work area) and gives your hook a call if it changes. Intended use case is plugging/unplugging a monitor.

Require or use-package this. Make a hook function which takes one argument, a pair of pixel width and height, like (1024 . 768), then add your hook to dispwatch-display-change-hooks. You will get called when that changes, eg by removing or adding a monitor. Then call (dispwatch-mode 1) to get started and (dispwatch-mode -1) to stop. (dispwatch-mode) toggles.

(defun my-display-changed-hook (disp)
  (message "rejiggering for %s" disp)
  (cond ((equal disp '(3840 . 1080))   ; laptop + ext monitor
         (setq font-size-pt 10))
        ((equal disp '(1920 . 1080))    ; just laptop
         (setq font-size-pt 11))
        (t (message "Unknown display size %sx%s" (car disp) (cdr disp)))))

(use-package dispwatch
  :config (and
      (add-hook 'dispwatch-display-change-hooks #'my-display-changed-hook)
      (dispwatch-mode 1)))

• This could be included as part of ewmctrl, more generally saving and restoring different sets of window configurations as the geometry changes. I have an open issue with those guys to discuss. Alternatively, this package can also stand by itself and be used by a window saver/restorer.