It is one size fits all.  I am a large and while I think it is okay, I did not like it because even in a light shower, I don't think you would get good coverage on the sides.  The sides are pretty much open to the elements from the waist down.  In rain/wind situations, I am sure you will have to hold on to the sides of the poncho to keep from getting wet.  I bought this and another poncho from the same seller.  I liked the other one better (and was thinking about buying another one to keep in my car).  Unfortunately, even if I don't care for this one, I'm going to have to keep it, too, because they are charging me for the return which comes to 30% of the purchase price--ridiculous.