Mainly depends on sweater style and people's posture. Simply divided into three categories.

1, knitting a slim sweater requires 6-6.5 inches from top to bottom.

To knit a slender sweater, you need to knit about 7 inches from top to bottom.

3. If you are knitting a loose sweater, or you are fat, you need to lengthen 1- 1.5 inches from top to bottom.