Although he is the most important character in To Kill a Mockingbird (aside from the narrator, Scout), Atticus is never described in great detail. As the previous post mentioned, Atticus is rather tall (probably over six feet), has black hair that is turning grey, and wears glasses because of an inherited family eye problem.
He was nearly blind in his left eye, and said left eyes were the tribal curse of the Finch family. Whenever he wanted to see something well, he turned his head and looked from his right eye.
Despite his age, which was older than most of Scout's fellow students' fathers, Atticus must have been in relatively good shape, since he rarely drove his car, preferring to walk each day to his office. During the trial, Scout tells the reader that Atticus never perspired, but now his face "was shining tan." Normally, Atticus never "loosened a scrap of his clothing until he undressed at bedtime." But just prior to his final summation,
... he unbuttoned his vest, unbuttoned his collar, loosened his tie, and took off his coat... this was the equivalent of him standing before us stark naked.
He is old and hair is turning grey as he ages. He is tall and wears spectacles.
Atticus is characterized as a older male, with a darker hair color that is turning gray as he ages. He wears glasses, as mentioned previously in answers to this question. He is also rather taller as though of in the reading.
Atticus is in his 50's, wears glasses, is tall with gray and black hair and a square face.