The project required a map with several tile layers with different tile sizes. Is it possible to do it within the editor’s capabilities? Or do I need to make several files-maps?
It is currently not possible.
If you have tiles that are orthogonal and their sizes are multiple of each other (e.g. one tileset is 16x16 and another is 32x32), you could load them all as the common factor size (in my example, that would be 16x16), and use the larger tiles as metatiles. It would be more of a hassle and you’d lose out on being able to use features like terrains though.