Unlike traditional patterns with a set gauge, a custom fit sweater gauge is your gauge, so the swatch contains lots of essential information. In fact, the pattern can't be generated without the swatch details.
A generous swatch is important for lots of reasons. A swatch is your first date with your yarn. You have a chance to see how it behaves on the needles and whether you like knitting the yarn as much as you liked looking at in the skein. Give yourself enough time with the yarn to make an informed decision on it. Do you like the feel of the yarn as you knit it? Do you like way the fabric feels and looks at the gauge you've knit it?
A nice large swatch will also give you a sense of how the yarn looks in a fabric at sweater scale. You might adore a busy, high contrast yarn in a small accessory, but you may not like it for an entire sweater. Or you might love it. But you won't know until you see it in a generous swatch. Before you commit to a big pile of yarn for your Custom Fit sweater, or any sweater, invest in a single skein of the yarn you're considering, and take it for a test drive.
Knit up a big swatch and then treat is as you will treat your finished garment. Going to wash it in the machine? Then wash your swatch. Same with drying. Most superwash wools really need a trip through the dryer to return to their original elasticity. If you are a dry flat kind of knitter, then do that. It's essential to know how your finished garment will look after it's lived in your world and been washed and worn as many times as I hope it will be. Get the scoop on your yarn at the start for a perfect sweater that stays perfect through many wonderful years of wear.
So, how do you make a perfect swatch for your Custom Fit sweater? As I said above, size matters, so your swatch needs to be at least 6 inches square or larger. Here's our recipe for a perfect swatch.
Stitches cast on
Cast on number of stitches for the gauge of your yarn.
Work designated number of rows garter stitch (knit every row)
Switch to stockinette keeping edge stitches at each side in garter stitch (border stitches indicated are on each side, so for worsted, you will keep three stitches at beginning and end of every row in garter stitch)
Knit indicated number of rows in stockinette (knit one row, purl one row)
Work indicated number of rows in garter stitch (knit every row)
Bind off loosely.
Wash and dry your swatch as you intend to wash and dry your sweater.