Posts from How do you create maps with non-standard dimensions?

I always thought that a map could only have a size ranging from S to G, but recently I found maps with only 18*18 and 6*6 cells. Where and how can I set an arbitrary number of cells for a map?

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