I calculated that my next wife's body mass index is 15.5, which is out of the range of 18.5-23.9. Which means my wife is a little thin. Now let me tell you the standard blouse size for women, hoping to help you:

1S 160/84

2M 165/88

3L 170/92

4XL 175/96

In view of online shopping, the size of the web page is the size of the tile of clothes, and the human body is three-dimensional, so there is a space of 3-5CM. In view of the thin figure and the thin jeans and shirts, I personally suggest that size L is better.