The advertised size of a bike is meaningless, because different companies use different systems. Before you go to a bikeshop, you need to know your best saddle height and best cockpit length (from the back edge of the saddle to the front edge of the stem). I know my saddle height is 30 inches and my cockpit length is 31 inches. If I can set up a bike with those measurements, it fits me like a glove, regardless of the advertised size (I have bikes "marked" as a size 17, and bikes marked as a size 24...each has a saddle height of 30 and a cockpit length of 31.
Take the bike to two different bike shops and have them dial in the best saddle height and cockpit length possible. If both shops say that it is impossible to dial in the fit for your size because the bike is too small, tell the seller you want to exchange the bike for one that fits you correctly. But, odds are, a good shop can dial in your new bike to fit you perfectly.