I think in terms of no. of actual initiates:

For the NPHC sororities: Alpha Kappa Alpha

For the NPC sororities:
Chi Omega, Tri Delta, Delta Zeta and then Kappa Kappa Gamma
(unsure about the exact order, but I believe this to be fairly accurate).

Also, there is a non-college sorority called Beta Sigma Phi, and it is supposed to be enormous, in terms of no. of initiates.

Hope this was helpful.