Recently, there was a great article posted on Quora on the size and layout of Alexandria, complete with science and calculations, of course.
Get your calculators ready because we’re going to calculate how many walkers it takes to wrap around Alexandria 20 deep.
In Season 6 Episode 5, Rick said that the half of the group of walkers was enough to surround Alexandria “20-deep”. If we assume that each “ring” of walkers has the same number of walkers, that means that there are 20 rings of 750 walkers each surrounding Alexandria.
Assuming that there is a roughly a distance of 3 feet between each walker (this roughly allows walkers to stand at two shoulder-width’s distance apart), Alexandria has a perimeter of 2250 feet.