Custom map size and tile size
I can't understand how to modify map size in terms of tiles, and tiles size: are they configurable? Can I have rectangular tiles rather than square tiles? I have 12x24 tiles and 10x10 maps.
I think you're talking about the tiny Isocat game. As I mentioned before Wright hasn't true support for isometric games so some fiddling may be needed.
The Isocat flat tile is 32x21 but "caged" into a 32x32 square. You can try fitting your 12x24 tiles into 24x24 squares, aligning the image to the top of the square, and then tune the the hitboxes to make them work.
About the map size, you should be able to create larger maps adding more letters to the map descriptors: https://github.com/kesiev/Wright/blob/master/tapes/isocat/scenes/stage1.json
Keep in mind that the Isocat game doesn't support scrolling camera.