I make them in GIMP, the image size is always 1920x696. Keep in mind, in a monochrome map, you have a pallete of 2 colors, black (blank area) and white (land), so If you want to texturize a map, get some patterns. I recommend you start by drawing out where you want the terrain by using 2-pixel radius brush as a border, then the inside with 1-pixel checkerboard. Also, watch out for automatic girders, they may appear even if you don't want them.
Also, when making patterns, test them, see how petrol flames behave on it and see if you can blowtorch correctly in them.