Tiny

This character is tiny compared to regular humans, small enough to fit into the hand of a regular human. This is an example though, actual size may vary.

While fairies are usually depicted at this size, the trait is not exclusive to them nor applies to them all. There may be many other fantastical explanations, like the potion Alice drank in Alice's Adventures in Wonderland that made her tiny enough to fit a very, very small door.