Most NPC orgs list the number of chapters on their website. Some just list them all and make you count them. Generally, when people say "largest" they mean "greatest number of currently active chapters." Of course, all the NPHC sororities are larger than all the NPC ones, in terms of number of chapters although not by numbers of initiates.
Ariesrising has a list at http://www.geocities.com/ariesrising...k/numbers.html
It's not "official" but she does puts in a lot of effort, so I'd bet her numbers are fairly accurate (give or take the constant fluctuation of number of chapters for any GLO). Chi O is the biggest both by chapters and initiates, and Theta Phi Alpha is the smallest.