I was looking at how to make Chrome extensions, and I got the idea to turn Deep Thought's javascript bookmarklet to fix the codebox in a spoiler size issue into a Chrome extension. It loads the javascript after the page finishes loading, so you don't need to worry about clicking a button or finding the bookmarklet in your list/toolbar.

So, here it is. It works for me, so I'm hoping it'll help some other people. Credit for the javascript powering this thing goes to Deep Thought, of course.

That's odd. I just removed the extension and restarted my browser to ensure there weren't any problems on my side, and the codeboxes in spoilers were tiny for me.
Then I reinstalled it, restarted Chrome again, and it works properly.