The tiniest letterbox?
Consider this front door, already one of the narrowest you will find that is still possible to get through. Consider next, that it's what is known as a storm door; the type where you only need open one half. Assuming you are very slim.
What looks at first sight like a small letterbox, is actually an ultra tiny one.
When you are delivering literature door to door (for the freedom of Toyland, obviously), you find yourself paying unnaturally detailed attention to steps, kerbs, cobbles, all of which you can measure your length over. And you develop a healthy respect for the infinite variety of letterboxes, all of which are lying in wait to skin your knuckles. Especially this one. Bless it!