Two things will make your sweater fit the way you expect it to.
1- Choosing the correct size.
2- Getting the correct gauge.
Choosing the right size, but guessing about gauge, will often lead to disappointment.
I know that gauge was a subject that I avoided when beginning knitting. Sounded way too complicated! But then I learned that it was as easy as using a different needle size!
The photo to the left shows how to measure gauge. I use the ruler with the little window, so that it is easier to see half’s and quarters of stitches. If you are knitting too loose or tight, even a tiny bit, change your needle size. A larger needle will make your gauge looser, a smaller needle will make it tighter.
Make sure you do your swatch in the round if you will be knitting in the round. Most knitters purl looser than they knit, so a swatch that is purled and knitted is often looser than one knitted in the round.
A small difference in gauge will make a LARGE difference in the finished sweater.
200 stitches knitted at 4 to the inch will be 50 in. around.
200 stitches knitted at 4 1/2 to the inch will be 44 in. around.
So if you do a swatch, measure carefully, you will be much happier!