I normally wear a size 3 in jeans and i want to get some sweats. However most sweats have the size S, M, and L. Which size should i get?

Also I bought some jeans and got a size three and they are still a bit baggy in the waist. Do you think I could go down to a size one w/o them being too tight. My problem is that I have big hips and a small waist.

My measurements are 25inch waist and my hips are probably 36 inches. Should I just stay with the size three or gone down?

Thanks for the answers!