If you've ever seen the film version of To Kill a Mockingbird, it's tough not to picture Atticus as the actor Gregory Peck. Peck is actually a good fit with Harper Lee's character. In the novel, Atticus is described as tall (he is a foot taller than his brother, Jack) and wearing glasses, being nearly blind in his left eye. In Chapter 10, Scout calls him "feeble" and "nearly fifty"; he is older than most of Scout's classmates' fathers, and Atticus was 15 years older than his late wife. Atticus is trim and fit, though he gets most of his exercise by walking to work each morning. In Chapter 20, Scout describes Atticus' dress: Being an attorney, Atticus always wears a suit (or coat), vest and tie. "He never loosened a scrap of his clothing until he undressed at bedtime." He carries a pocketwatch which he often rubs so he can think more clearly, and he wears a hat outdoors.
This is a good question. To be able to picture someone makes you enjoy the book so much more. Unfortunately, there is no clear description of Atticus in the first five chapters of the book. The beginning of the book describes his family lineage instead. For example, the Finches have been in Maycomb from time immemorial.
For a sustained physical description of Atticus, you need to go to chapter ten. In this chapter, Atticus is described as nearly blind in one eye and bookish. Here is what the text says:
Besides that, he wore glasses. He was nearly blind in his left eye, and said left eyes were the tribal curse of the Finches. Whenever he wanted to see something well, he turned his head and looked from his right eye.
He did not do the things our schoolmates’ fathers did: he never went hunting, he did not play poker or fish or drink or smoke. He sat in the livingroom and read.
For more on the description of Atticus, you need to piece things together, such as when Scout says that Atticus was tall, as tall as Heck Tate.
I know that this is limited, but that is part of the beauty of the book. Atticus is described not so much by his physical appearance but his character - courage, integrity, and sense of justice.