All storefront doors are comprised of two aluminum vertical and two horizontal rails that are held together by tie rods or screws.
The left and right vertical of a narrow stile door is approximately 2" in width. The top horiziontal, "referred to as a top rail" is approximately 2 1/8" wide, and the bottom horiziontal, "referred to as a bottom rail" is approximately 3 1/4" wide; however, the bottom rail does come with different options EX: ADA compliant door needs a 10" bottom rail and www.StorefrontDoors.NET can provide you with this if needed.
In almost all cases a narrow stile storefront door will work for your storefront project unless you've got a lot of traffic and then you may want to consider a medium or wide stile storefront door.
Some architects will choose a medium or wide stile door for your new building for architectural viewing purposes.
More on storefront door stiles here: Storefront Door Stiles