Our Standard Fit shirts and polos are trimmer than most on the market, and our Slim Fits are a bit more tailored than that. The new Standard Fit polos will fit identically to the older Athletic Fit polos.
As a general rule, go with your normal t-shirt size (either at Bonobos or other brands). If you''re skinnier than most guys, get the Slim Fit. If you're comparing to your measured chest size (measure at the biggest part of your chest), the chest of your shirt should be between 2 and 6 inches larger than that of your body.