I paint in oils. I use small brushes and do very detailed work, but I work on good-sized canvases. Just viewing the itty-bitty portraits gave me eyestrain.
However, large paintings are not very portable, which is why clever artists developed miniature paintings. Artists painted these teeny little likenesses from the 16th century up until 1839, when Louis-Jacques-Mandé Daguerre showed off his invention to the members of the French Académie des Sciences and photography was born. You can read about Daguerre (1787–1851) and the Invention of Photography at the Metropolitan Museum of Art's website. Naturally, Daguerre named his invention after himself calling them daguerreotypes.
You no longer have to travel to Washington, D.C. to see miniature portraits. There's a great collection online at the Cleveland Museum of Art as well as the one at the Smithsonian. I've found plenty of these treasures on Pinterest, too. I think I'll start another board of miniatures there.
In my soon-to-be-released book, Patriot's Heart, the heroine gets to see a miniature of her mother--a very small portrait which fits in the palm of her hand.
Maybe I got the inspiration for that scene at the Smithsonian back in 1991. :-)