I think it depends on which fit it is. They have two major styles, "the hugger", and "e-z ". I wear a XL in the EG work shirt, and XL in Gant Rugger "the hugger" shirts (a relaxed/tailored fit. Not tight in anyway, but well fitting)
. I would probably wear a L in say RL "custom fit", and a 16/16.5 -36/37 in US sizing.
Im a bit unsure of the "E Z" fit, which is less slim and has wider shoulders/shorter sleeves. The "Hugger" fit is probably my favourite shirt fit allaround, recommended.