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. 1×1, 2×2, 3×3, 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