Ok I understand what you are saying now ... besides the requirements I listed below from our website, there isn't really a perfect size but rather what the artist prefers. If you would like some guidelines, I would recommend the following:
Square: 2,048 x 2,048 (ratio 1:1)
Portrait: 2,048 x 2,560 (ratio 4:5)
Landscape: 3912 x 2048 (ratio 1.91:1)
Please let me know if this helps or if you have any other questions. Thanks! - Joe @ NeonMob
NeonMob Image Requirements: "Cards should be exported at a minimum of 1024 pixels for height and width, but bigger is generally better. Images may be portrait, square, or landscape, although collectors have shown a slight preference for portrait shaped cards. The maximum width-to-height dimensions allowed are 2:1 or 1:2."