ANDREA POLIZZI FOR REX LESTER Black dress with keyhole back

This is an Andrea Polizzi for Rex Lester black dress with no size, however I am a size 4/S/27 and I could barely fit it over my hips. I am assuming it is a true size 0 or 2. Purchased at a vintage thrift boutique (foolishly without trying first) Slight pull at the zipper but nothing major