Hello, about sprite sheets, so for the moment only columns number can be defined. So to have all in one row left to right, you can put the number of images in columns number. Or you can just put super high number to be sure (in case of several animations), it will be cropped to the necessary size for each sprite sheet anyway. But yes this could definitively be improved indeed.
For aseprite export, you can just choose aseprite extension and that's it : 