They do come in varying sizes, although I can't remember seeing a label on them or anything. They tend to be quite big as they are designed for wearing in a cold enviornment over insulative layers.
If you wear woolen clothing (which I strongly advise in the British climate) then you will find that a swedish snow smock will keep you dry when worn over a swanndri type shirt in a moderate shower. The Swanndri may get a little damp, but you won't get wet through for a long while. Adding a water proofing agent, such as cotton nikwax or greenland wax as Addo has will only make it better in wet conditions.
I wore my smock in a downpour for an hour talking to an aquaintance at the bottom of my road. No tarp over head, no trees to shelter me. My hat got wet on the outside (the woolen one I got from Canada) but my head stayed dry, and my smock wetted out and cardboarded a bit, but I was dry underneath and didn't feel uncomfortable at all. If I was caught out outdoors on a tramp, I'd have thrown my poncho on over the top.
