A relatively small overall body type.
Tall women are not petite even if they are slender. Such images should be tagged skinny.
Petite should only be used for people of natural size. An exceptionally small person should instead be tagged minigirl or miniboy.
For female characters that appear fully underaged, use child if the image is safe and loli or shota instead if it is questionable or explicit. See howto:rate for more.