Largest is kinda hard to pin down because the organization of sections is pretty loosely defined. Since all you need is two or five NAR members to have a section (two to start, five to maintain), once that threshold is met it doesn't really matter. Historically, I think DARS (Dallas, TX) has been the largest in the past couple of decades, with a membership usually (afaik) in the 200's. I would think one or two of the California clubs might be as large. NARHAMS in Maryland has usually got a pretty steady number of people. My own section, SoAR (Atlanta) has 111 as current in the database, with probably a couple dozen extras who claim us on their NAR membership but haven't paid our dues this year. And since we don't require NAR membership to be a SoAR member, for the most part, the low power flyers tend to be unaffiliated. I
The smallest sections are probably those NAR President John Hockheimer referred to recently as "private NAR Sections". Small groups of people that become a section primarily for the site insurance, and otherwise don't advertise for members. So by definition that would be the 2- and 5-member sections.