Here are the sororities ranked roughly by size (number of chapters currently active, not initiated women or number of chapters ever installed)
176 chapters - Chi Omega
More than 150 chapters - Delta Zeta, Zeta Tau Alpha
More than 130 chapters - Delta Gamma, Alpha Phi, Delta Delta Delta, Alpha Delta Pi, Alpha Chi Omega
More than 115 chapters - Alpha Omicron Pi, Pi Beta Phi, Kappa Delta, Phi Mu, Kappa Kappa Gamma, Kappa Alpha Theta
More than 100 chapters - Gamma Phi Beta, Sigma Kappa, Alpha Gamma Delta, Sigma Sigma Sigma, Alpha Xi Delta, Phi Sigma Sigma
More than 50 chapters - Alpha Sigma Tau, Alpha Sigma Alpha, Alpha Epsilon Phi, Phi Sigma Sigma, Delta Phi Epsilon
50 or fewer - Sigma Delta Tau, Theta Phi Alpha
This is an old post by Fuzzie Alum from a thread back in February. While there ARE going to be slight changes in numbers and categorical shifts, this is the approximate break down.
PMMama---I believe Phi Mu is around 121 or so active chapters at last check. I'm not sure on the number of initiates though. I will admit we are much larger in the south, but I generally wouldn't say that we are a "small" group. (Not that there's anything wrong with that though)
Hope this info helps!