Our practical gilet is windproof and waterproof and once you have worn one it will become your favourite bit of kit. It's made from two layers of microfleece with a waterproof film (Hytrel from DuPont)bonded between the layers. It has supplex side panels which allow for ease of dressing and undressing (even in a single) and has supplex panel pockets on the back for waterbottle, pogies and stuff!
It is cut short at the front so that it doesn't fold over when you are sat in a boat. It should not be baggy so it can't trap your thumbs at the end of the stroke but mustn't be tight under the arms.
As a guide - the xs of these fits size UK8-10 women, medium UK12-14 women. Most women want a size smaller than they expect and most men a size bigger! The supplex side panels mean that they are less tightly fitting than the activity fleece.