The numbers in the sizing chart take one layer of insulation into consideration; if you fall in the upper range of a size, it is recommended that you choose the next size up. You can put on the maximum number of layers you plan to wear for the coldest, foulest conditions you ever plan to be in and adjust to the chart accordingly. Take measurements with insulation on. The sleeve is measured with the elbow bent, thumb 6" from chin, and measured from the center point of the back of the neck around the elbow to the wrist bone. The inseam is from crotch to ankle bone. The chest is around the fullest part of your chest. The waist is where you normally wear your pants.

A NOTE ON DRY SUIT SIZING
A common mistake with dry suits is making a judgment about the size by looking at the garment while still on the hanger. A suit on the rack always looks bigger than when it is on you. If you fall on the high end of the size scale, move up to the next size and consider shortening sleeves and legs (dry suits only).