I usually wear a 3X in normal clothing. In a Performance jersey I usually buy a 3x and they are a bit tight. I have recently switched to buying all my cycling gear from Love2Pedal and I buy a 3x or 4x jersey. The 3x is just a bit snug and the 4x is plenty roomy so I would say I am at a 3.5x

I ordered the 5x based on laying out my most comfortable jersey and measuring it from armpit to armpit. I'm glad I did go with a size bigger. It is just about the same fit as my 3x from Love2Pedal. The 5x is certainly not too tight but it isn't any too big either. I don't know that a 4x would have worked for me.

(No pics coming so don't hold your breath!)