Submissions for donation have to be within a certain size range to be accepted. The size to aim for is one that would fill most of a signature, about the size of Spotty's usual donations or the Lenval puppy lineart. If it's much smaller or larger than that, it will be either refused or resized. Also, be sure the design is fully shown on the lineart submitted - generally a slightly angled shot that shows the side and face is best for displaying designs.
