Both ATCs and ACEOs are 2 1/2 x 3 1/2 inch works of art. The only real difference between the two is the commercial (for sale) intent of the art piece by the artist.
ACEO = Art Cards, Editions, and Originals are virtually the same as ATCs, EXCEPT they were created with the intent to be sold
ATC = Artist Trading Cards were created with the intent to trade them only.