I recently bought this item, a 36, which I assumed was small. I expected it to be generous so I ordered a size smaller than I normally would have. It fits perfectly so I would say they are a little generous with their sizing. I found this true of another company I ordered a similar sweater from.
I normally wear a size 4 or 6 top, or a size small. I have this is both the size 34 and size 36. Both work well for me, depending on the weight of blouses or sweaters that I am wearing. I would say that the 38 is probably equivalent to a size M.