I have the 54 mm large Ray Ban 2140 wayfarer. I purchased it after i saw Ashley Olsen and Mary-Kate Olsen wearing them everywhere. I don't like the wayfarers they stock at Sunglass Hut, because they are usually the small 50 mm ones that everyone has and they seem more hipster to me.
If you have a small face then i suppose the 50mm would look perfect. But I like the 54mm ones because it looks chic and still has that great oversized feel and oh so very Audrey Hepburn.