Where would modern-day Maycomb be located?
Maycomb is based on Monroeville, Alabama. The map here shows where Monroeville is:
Here is background on Monroeville
Here's a fun hand drawn map of Maycomb itself: http://zsp417.tripod.com/id4.html
The web site epodunk.com gives a profile of the fictitious town of Maycomb, based on the author's hometown, Monroeville, Alabama. Interestingly, the town of Monroeville has a mall called Maycomb Mall in recognition of its ficitional namesake.
Although Harper Lee based the town on her own hometown, Maycomb could have been any small town in the south of the 1930s. The attitudes and injustices of the era need to be emphasized more than place.
Monroeville, Alabama is the place most people agree is the fictional Maycomb. It sits in south-central Alabama, about 162 miles south of Birmingham. This is the town where Harper Lee grew up. Like many authors, she used what she knew and was familiar with as a setting for her story.