Hmmmm....here's my best stab at how to proceed....
In the pattern for the sweater, there should be a gauge listing---stitch counts across and stitch counts up/down for 4". Divide this number by 4 to get stitches per inch. Read the pattern also for a description of the yarn weight---worsted, DK, etc---and recommended needle size. Those 3 pieces of info you need when you go to your LYS: stitches per inch, yarn weight, needle size.
Then go to your LYS, pick out some yarns of the right weight and a similar # of stitches per inch and needle size---that info is on all yarn labels. Then ask the a nice LYS person's opinion of the yarn you selected, based on the pattern requirements.
Then, start knitting gauge swatches. If you can't match the gauge of the pattern, then you can't accurately predict the size and shaping of your garment. If you use the recommended needle size & your swatch comes out with too many stitches per inch, then knit another gauge swatch with a larger needle size.
In my opinion, it's more important to meet gauge across the garment than up & down the garment---if you need to adjust the math for # of rows, that's easier than adjusting the math for # of stitches.
BTW, in order to really know how the yarn is going to wear when you're finished with the sweater, you should wash your swatch to see if washing & drying alters how the stitches relate to one another---it could loosen or tighten, or do something wacky 'cuz it's got lots of personality!
Hope that helps