I have been painting "Pocket Art" for years. These small paintings are known as Artist Trading Cards or ACEOs.
An article I wrote about this art form can be found here. The basic rule for an art card is it must measure 2.5 x 3.5". ACEOs can be sold or traded. If the card is a print it must say so.
I find a real joy in painting these small pieces. It is very satisfying to complete an entire painting in an evening. It is also a wonderful way to work out composition and palette in preparation for a larger work.
To date, I have sold over a thousand ACEOs on ebay (look for auctions by KSHpaints) and enjoy hearing from my collectors.
Here is a selection of ACEOs I have painted. All are 2.5 x 3.5"