For a size comparison, check
this blog post. And
here's a shot of Caesar Miniature's Modern Militia alongside a Matchbox car. The figure on the front left is supposed to be a child soldier - he's smaller on purpose.
Somebody did a detailed analysis of various scales for Hot Wheels and Matchbox cars with 20mm and 1/72 figures. I just saw it last week, but damned if I can find it again. Anybody know what I'm talking about?