In most cases. women's wetsuits are sized in dress sizes. (2-6-14-etc) Men's are listed in "sizes." (S-M-L-etc) Some unisex type suits come in sizes, but they will also, at times, show a chart that will have height/weight so you can determine the size you need. I would suggest always trying on a wetsuit before purchasing.
As for what are they made of: (from Wikipedia) Wetsuits are made out of closed-cell, foam neoprene, a synthetic rubber that contains small bubbles of nitrogen gas when made for use as wetsuit material. Nitrogen gas has very low thermal conductivity, so it does not allow heat from the body (or the water trapped between the body and the wetsuit) to be lost to the water outside of the wetsuit. Some wetsuits of late have even begun to incorporate Merino wool and titanium fibers to add warmth characteristics, while keeping the thickness of the suit to a minimum. A wetsuit must have a snug fit to work efficiently; too loose a fit will allow water to escape from between the suit and the body, taking the body's heat with it. Flexible seals at the suit cuffs aid in the water retention. Neoprene is very buoyant, helping swimmers to better stay afloat, and for this reason divers need to calculate extra weight values based on the thickness of their suit to achieve neutral buoyancy underwater. The suit loses buoyancy and thermal protection as the bubbles in the neoprene are compressed at depth. | 
How do you make sure you have the right size wetsuit 
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