A relatively small overall body type; low body mass. Height is to be included, thus tall women aren't petite, even if they are slender. In this case they should be tagged skinny. Note that having large breasts doesn't disqualify. When there is a focus on the difference between a petite girl and someone much larger, it is a case of size difference.
An excessively small minigirl or miniboy is generally considered to be something different; petite is usually only used for people of natural or roughly-natural size.
Petite characters are most of the time small young adult women who look way younger than their real age. For actual underage female characters use child (no sex) or loli (sex) instead.