Did you make a gauge swatch before you started knitting?
The gauge for the pattern you are working on is 20 stitches to 4 inches on a size 6 needle. My yarn gauge ruler measures 2 inches so I usually cast on the required number of stitches and measure for 2 inches – if you measure across 2 inches of knitting you should have 10 stitches on your needle.
If you don’t get enough stitches for your gauge you need to drop a needle size or 2, too many stitches and you need to increase your needle size. Then after you have figured out which needle gives you the right gauge, you can cast on for the sweater.
Are you using the right weight yarn? You should either be using the yarn recommended in the pattern or you should be using another yarn of a similar weight. Any worsted weight yarn should get you the right gauge for that pattern
A size 16 needle (there is not one, only 15 or 17) is a very large needle and you wouldn’t be using a needle that large for your project so it’s not a typo.
The sweater you are making is knit in the round so you should be making your swatch by knitting across the row and then slipping the work over to the other end of the needle. Leave a large strand of yarn around the back (almost like you were making an I Cord but not pulling tight) and knit back across the row. I know there is supposed to be an explanation of how to knit a gauge swatch in the round but I can’t seem to find the link or make it work.
Hope this helps.
Best of luck,