As someone who's not a size 2, I love these dresses from this thread (and am sorely tempted by several of them):
But this is definitely my fave, it looks ridiculously flattering for several body types:
OP, you may also want to double check to see whether wearing black to Pref is recommended or discouraged at your school- that'll narrow your choices down substantially. That info may be on the Panhellenic website, or you can try calling the Greek Life office.
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And in the years after, with tears or with laughter, we'll always remember our dear Kappa days.