If you have not yet heard of art trading cards or ACEO’s, here is a bit of history courtesy of wikipedia.
The dimensions of Art Trading Cards are the same as modern baseball cards. They are miniature works of art, 2.5 inches by 3.5 inches, originals, and sometimes limited edition prints.
The cards are usually traded or exchanged, rather than sold. This one was made as a free enclosure in a gift certificate, sent to newly weds for a custom painting from my Etsy shop.