How do you create maps with non-standard dimensions?

I remember reading somewhere about how to do this, but unfortunately, I don't recall where or how. It seems you can create an empty map, save it, and then use a HEX editor to change the bytes that specify the map size in cells.
But here's a secret: you can actually create a map of any smaller size in the standard editor very easily. All you need to do is outline the desired area with a moat of emptiness (or, conversely, fill the entire level with emptiness and create a small square of grass/snow in the middle...). And you'll get a map that's even 2x2. :D