Atc's or artists trading cards are 2.5 inches by 3.5 inches.
I recently made a couple of small books using them as hard covers and wanted to make some more for friends but I need to research how you make a hard covered book with a spine. These are all acrylic - subtraction pears and acrylic pours.
In case you were wondering how you do tiny acrylic pours, you use a tiny disposable shot glass! I found I didn't get many cells this time which was a shame - I think the paint was too thick.