do a search on this topic and you'll find a million threads. The consensus is basically:
1. Depending on whether you're talking about number of initiated members, number of chapters, and a lot of other factors, the largest sorority can vary. Also, many NPHC groups are a lot bigger than NPC groups, and some non-collegiate sororities are very large too.
2. A lot of us on GC feel that discussing this topic (it comes up every month or so) is somewhat defeating our purpose, which is promoting pan-Greek feeling. It's not the size but the siblinghood that matters.
3. To get the information you want, I recommend you do a thread search and visit the websites of various national groups.