So 1080x1920? Or 720x1280?
You're free to do as you will, but with mp3tag you can mass adjust embedded album art sizes; as well as mass extract artwork if you wish to keep the higher resolutions on-hand.
I would recommend not having album art sizes that exceed the smaller of the two dimensions of your display, be it the horizontal or vertical pixel count (so no higher than 1080x1080, or 720x720).
Example:
1600x1600 = 2,560,000 pixels
1080x1080 = 1,166,400 pixels
720x720 = 518,400 pixels
For detail that the display cannot resolve, album art is taking more than double the memory, if not 5 times the memory.