I used to try to size legs to "fly size". But then I noticed something - the small [and partically the x-small size] often "glued" themselves to the fly body when twitched forward. Then just stuck and, of course, gave no further action. The med legs are stiffer and give you good movement which is why we use the rubber legs. Now I no longer try to use rubber legs on really small flies. I just different leg material - soft hackles, for example. I will still sometimes use the rubber legs for "tails".

Donald