Like many characters in literature, Ellie mostly just "looks" the way you picture her. However, Woodson does provide some information and clues to her appearance.
Ellie is white. This is important to the plot since much of the story revolves around her interracial relationship with Jeremiah. Ellie is also Jewish. Jeremiah views her as beautiful, which the reader can also easily picture when considering her from his point of view.
In the cover art of a current publication of the novel, Ellie is pictured as a slender girl with long, dark hair and dark eyes. She is a teenager and can often be seen in her prep school uniform, which is pictured as a white polo and gray pleated skirt, typical of the kind of high school she attends.