I saw that they implemented that option, is it possible that over time they add the option to set the resolution? That is, I download songs in which the covers are saved with the 1280x720 cover and with the option they added it could be set to 500x500, I saw that it only lets you set a value.
You ask if it is possible to stretch or squeeze a cover picture disregarding the aspect ratio?
This is not possible with Mp3tag.
The help documentation about "Adjust Cover" says:
Max. Size is the maximum width and height in pixels of the embedded cover image. If the cover is smaller than the size provided, the size of the cover is not changed. Resizing the image keeps the aspect ratio.