It is difficult to imagine Atticus as looking much different than Gregory Peck, who memorably portrayed him in the 1962 film adaptation of the book. But Scout describes Atticus in a few instances. She says that he has black hair, that of course turns increasingly gray throughout the book, both in response to advancing age and no doubt to the stress he is under as a result of the trial.
Above all, she says he is "feeble;" Scout and Jem both think of him as being very old, far older than the fathers of their friends. They do not think of him as physically imposing in any way. He is nearly blind in one eye and wears glasses. He is somewhat bookish and unassuming, and he does not fit their model of masculinity. He does not, for example, play football with Jem. This is why the children are so surprised when Atticus takes down Tim Johnson, a dog that has gone mad with rabies, with a single shot from a rifle.
In chapter 15, Scout contrasts Atticus's appearance with that of Jem, whose "soft brown hair and eyes" and "oval face and snug-fitting ears" are inherited from his mother. Atticus, she says, has "graying black hair" and "square cut features," even if they both share a resoluteness and "mutual defiance."