This is how my chapter did it this past year:
1. Sisters got the same size shirt they ordered for the Panhellenic issued recruitment shirts for first round.
2. For new members, we ordered an additional 25 shirts (even though quota would be much smaller than this) and just kinda did a guessing game....more than likely we'd have women who'd want larges or XL's, so we ordered more of those than the others. We ordered a couple mediums and a couple of XXL's for the "larger" women, and that was that. We also found a GREAT company who was able to do a rush order for a really reasonable amount of money!
Hope this helps....it really is a guessing game when it comes to ordering shirts for new members!