Measure your favourite piece of well fitting, not-too-stretchy clothing and compare it to the size chart below. Just make sure you have enough room for thermal underwear layers and a hearty lunch at the snow :)
Really good fit for me. I am normally a 3XL but went for a 4 XL as I had been told this brand is a "small" 4XL. Also the service I got from PlusSnow was really good - responsive (to my messaging on a couple of issues) and quick delivery.