These are definitely pants, not leggings, but they're close-fitted enough and a light enough weight to wear like leggings or breeches.  Unlike the denim leggings I bought from the same manufacturer, these have quite a bit of give around the knee, calf, and ankle, so, if you're looking for something that's going to fit like tights?  This isn't it.  Which makes me and my chicken legs very happy.

Small is a size 3.  I don't do enough shopping to know what's a true size anymore, but I usually wear a size 2 or 3.