The very popular size of ATC's (artist trading cards) 2.5 x 3.5 ''
This one is oil on 140 # watercolor paper, with a small white border.
ACEOs are miniature pieces of art that measure 2-1/2" x 3-1/2". If a small piece of art has different measurements than this, it does not qualify as an ACEO.
The acronym "ACEO" stands for Art Cards, Editions and Originals. They can be original art, print editions or photographs.