Scout is quite a complex character because of the depth she has. When the novel begins, Scout is a six-year old tomboy in a sleepy Southern town. She is beginning school and learning how to get along with the residents of Maycomb and establish her position in the community. She is a curious, determined and passionate young girl.
Internally we have an adult character who is actually reflecting on the experiences of herself as a six year old during a time of great social and political significance. Just as Scout physically grows during the course of the novel, we see her view of others mature through the gentle teachings of her father and the brutal lessons of the two deaths she encounters -
'Atticus, he was real nice...'
His hands were under my chin, pulling up the cover, tucking it around me.
'Most people are, Scout, when you finally see them.'
Very little information is offered up about Scout's physical appearance through To Kill a Mockingbird, possibly because she is the narrator of the novel, and as a narrator, she is unlikely to spend excessive time describing herself. We do know that Scout is six years old at the beginning of the book and that she is slightly bigger than the average child of her age. She likely has an athletic and strong appearance, as we frequently see her engage in physical fights with boys older than her. We also know that she prefers to dress like a tomboy and despises her Aunt Alexandra's attempts to fashion her into a young lady by making her wear dresses.
It's impossible to provide a description of Scout's internal thoughts or feelings, as they are constantly changing as she changes and learns more about the world. In an overall sense, we could say that Scout is an introspective, observant, thoughtful, intelligent, and self-aware girl; her narrative voice certainly sounds older than that of a child.