This is about very small pieces of art, usually 6x6 inches (15 x 15 cm) or less. Often called Small-Format Art, they should be distinguished from "miniature" artworks because the latter use brush-strokes that have been scaled down in size to suit and the techniques employed are highly meticulous and painstaking.
Small Format Art may still be "miniature" in size, however. My own will go to 3.5 x 2.5 inches (ACEO art card size). Despite the small sizes, these can often take as long to do as a larger painting...I tend to find it is longer, due to the greater concentration required. I use oils or acrylics, occasionally watercolour or gouache as well. The paintings are done on various surfaces...I like to experiment, however I ensure that the technique is reliably sound before committing to a complete painting.
They may be on prepared mdf board, hot-pressed watercolour paper or speciality oil-paper, to name a few. When completed and available, I place them at Folksy.com, in my shop SmallArt Stuff.