I bought this according to chest size in inches. The small fit perfectly around the chest, shoulders and arms. It's not super baggy as some would think from Burton products, nor is it skin tight. The sleeve length is long enough cover part of my palm before stretching. But the length below the waist covers my crotch area. This is great for those who want full length coverage, but I use it primarily for cold weather working.
I've worn it for a whole work week (~6hrs a night outside) without washing and the thermal stays fresh, while I stay cheesy. It feels silky smooth and you hardly notice it there after a few moments. It's also pretty breathable so you don't feel like you're secretly making cheese under your shirt.