These are art cards...so much fun...they measure 2-1/2 by 3-1/2 inches and fit into baseball card sleeves. About 5 years ago I started noticing the label ACEO on some artist's listings on ebay. I investigated and found out what they were: mini artworks. Of course I had to jump right in and for awhile I was addicted to producing art cards. I have produced 100's.
I also collected a few myself from my favorite artists. On a rare occasion there is an artist whose cards sell for a lot of money, over $100. I am not one of those artists...and I don't buy from those artists. But I have had a lot of fun producing these cards.
I usually use colored pencils for these mini works and pen on a smooth vellum, cut to size with my super duper paper cutter. Sometimes I use watercolor on Arches cotton paper and once in awhile I produce a collage on mat board.
If you search ACEO on ebay you will see that there are hundreds of art cards listed for sale. Rarely do I see one that fits my taste, just showing how subjective taste in art is! I like clean and offbeat art usually. I love a good collage, not a sappy Victorian one with old-time ladies and old-time children...not to say they aren't wonderful for those who love that kind of art.