ACEO stands for Art Cards Editions & Originals. ACEOs are available for purchase. Many people like to collect these small, fun, affordable artworks.
Artist Trading Cards & ACEOs are usually 2.5 x 3.5 inches in size. To me, they are any works that size or smaller.
Now, many of my ACEOs are available in my Etsy shop. The ones that I make smaller than that are called WILD SIZE ones. ACEOs are usually 2.5 x 3.5 inches. That is a rule. I started to make works like this back in 2004 though, before I knew anyone else was doing this, and mine were whatever size I wanted them to be. I still don't think I'm "breaking the ACEO rule" since I hadn't heard there was a rule. As far as I'm concerned I made this up myself, and the whole point of what I was doing was to make really small things.
You can see my first ACEOs here. At the time, I called them Pocket Paintings & Dollhouse Paintings.