A quick and easy way to add curve hugging shape to a boxy sweater is to add ribbing. The question is, how much ribbing? Equal ribbing (ex. 1x1, 2x2, 3x3, etc.) is about half as wide as StSt in the same stitch count.
For example, if 20 sts in StSt is 4 inches wide, the same 20 sts would be 2 inches wide in equal ribbing.
x inches in StSt = x/2 inches in equal rib