Perspective match and scale

[tricky]

I'm trying to use the perspective match utility which seems to be working ok, in that I have managed to create the camera which seems to be the right focal length, but the scale of items that I add is way out. For instance, there are some railings in the scene that are about 1.1m high in reality, but when I create a post next to them (a thin box), the box is about 65m high. How do you fix the scale? I must be missing something obvious.