It took a few years but I finally learned the importance of swatching to get the right gauge before starting a sweater. They either fit nice or they don't if your gauge is off and the gauge varies between knitters and needles. I started keeping a notebook of swatches and washing results for later reference and I am finding out I need to keep more complete notes, what needles did I use? what is the gauge? did it shrink? fiber content? which pattern? how many balls of yarn did I use? All these pressing questions for the next sweater.
I usually knit a swatch 8 - 10 stitches larger than the gauge so I have 2 or 3 garter stitches on each side so it doesn't curl then I can measure between the garter edge. It is also important to block the swatch, give it a soak and let it dry then check the gauge. These fibers change after they get wet, some have a lot of sizing and they shrink in length or width so I might need to knit it longer to allow for that shrinkage. I trace the swatch into my notebook then compare. I toss it in the washer with similar fabrics and check if it shrinks in the wash. Don't you hate when that happens to your jeans especially during lockdown this past year. Then into the dryer on low for a few minutes check and compare eventually drying on medium just to see what happens.
If I have a lot of yarn left over I will knit a washcloth size swatch because that will really tell me how much it is shrinking in the wash or the dryer. A quarter inch on a 4 inch swatch turns into almost an inch on a 12 inch swatch which could be 3 or 4 inches of shrinkage on a sweater. So sad when that beautiful sweater now fits a grade school kid. I will even run that larger swatch through the process a couple more times to see if its a once and done shrink or if it continues to shrink.
I know this seems like a tremendous bother but when I spend $60 to $120 on yarn for a sweater and many hours of knitting I don't want to ruin it in the wash. I always do this fussiness with plant fiber yarn because technically they are all water friendly even though the label says hand wash, dry flat, unlike animal fibers which will either felt with moisture and heat or in the case of superwash wools they will stretch then into the dryer with them until they are the right size again. Some fibers/yarns are worth the extra care and they feel so good.
I tape the yarn label and a piece of the yarn into the notebook and sew a label to the smaller swatch and save it in the swatch box for future reference. I might want to knit that yarn again. Happy swatching and happy fitting --Ann--