I'm doing the most popular graphics used on Wattpad, so this is not the list of everything because that would take a while. This is the size in pixels width X height. For some of the graphics, other people may use different sizes this is just the most common size.

512x800

It's a book cover for Wattpad... I don't know how else to explain it

700x110 there are more sizes but this is the size that is most popularly used

As a side graphic in a chapter of a story

500x200

As a side graphic in a chapter of a story or a graphic to represent yourself. Things such as likes, dislikes, hobbies, and etc.

400x600 or 500x800

as a side graphic in a chapter of a story

1920x600

a profile header in your profile... I'm doing a very bad job at explaining these

300x300, 500x500, or anything that's a perfect square (both width and height are the same size)

A profile picture

Any size

A graphic that has things that explain a certain character in your story

500x700

To set the tone in a chapter of a book