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Unfortunately, I got my turtle when i was pretty young, so i almost solely depended on my parents' (very limited) knowledge of turtles to watch my turtle's health. Lately I have been doing more research though, and I learned that painted turtles' average size of their shell is 12 inches, while hers is only 4 inches long. I know that females are supposed to be larger than males. I keep her in a 20 gallon long (I know that is too small, but my parents don't believe me and I don't have the money to buy a bigger one). I am really nervous that her growth is stunted, but she is otherwise perfectly healthy. Is it possible that she is just very small or does she definitely have stunted growth? if so, is there a way to remedy this? (she's 7 years old)

I would think 12" is fairly large for a painted turtle. 4" is a bit small, though not that far off for a male painted turtle. Do not overfeed, even if you think the turtle is malnourished and just stick to a consistent and healthy diet.