Only three sororities in Chicago for you:
Kappa Alpha Theta
Alpha Omicron Pi
Delta Gamma
They will hold a formal rush when you get here where you learn about each sorority and where you fit in. It's really not about reputations, since all three are mixed but about your fit with the sisters. It's also a top school so everyone here has great grades. As for partying, none have a reputation for being party animals but each has their own events, fundraisers, events with fraternities, charity events, etc.
I would say that they each have between 40-50 girls, except DG is only a couple years old on our chapter and may have less. If you don't do formal rush but hangout with certain sisters they may give you an open bid later on in the year.
Here are some webpages:
http://alpha-omicron-pi.uchicago.edu/
http://deltagamma.uchicago.edu/
http://theta1.uchicago.edu/
http://greekcouncil.uchicago.edu/