There are many books and resources to help knitters wade their way through sweater styles and designs that suit different silhouettes. I recommend utilizing your public library for pattern and sweater fit books to research your body shape and sweaters styles that suit it and you. Creating a style vision board from magazines clippings or on online organizers are helpful in identifying personal style. Also, browsing finished object photos in Ravelry for people with similar body types to yours also helps with imagining how the finished sweater will fit you and what modifications, if any, you can make.
Another point to consider when thinking about ease is the weight of yarn you intend to use in your sweater. The fabric produced in knitting differs depending on the yarn and gauge. A bulky yarn creates a thicker fabric and thereby consumes some of the ease measurement, a thinner yarn will consume less. This means a bulky weight sweater with 1” positive ease may fit more like a sweater with zero ease because of the thickness of fabric.
Knit a swatch starting with a needle size two sizes smaller than the one recommended in the pattern. Cast on about 50% to 70% more stitches then the gauge measurement tells you in the pattern. For example, the gauge measurement reported in the pattern is 22 sts in 4 inches, cast on between 11 to 15 extra stitches. Knit in pattern that the pattern reports, in general this is stockinette stitch, but can sometimes be different. If your sweater is knit in the round then knit the swatch in the round. If your sweater involves colorwork, knit a colorwork swatch. Every 3 to 4 inches of fabric (or more if the row gauge matters to sweater size), increase your needle size until you’ve increased through a total of 5 needle sizes.
When your swatch is dry choose the fabric you like best of the differing needle sizes. Measure gauge by placing a locking stitch marker in between to visible knit stitches at the furthest left hand point possible. Using a measuring tape, measure 4 inches across the fabric and place another stitch marker at exactly the 4 inch mark. Count the stitches between the markers and compare the amount to the gauge stated in the pattern. If there are not enough stitches your sweater will be too big, if there are too many stitches your sweater will be too small.
If the fabric you like in a worsted weight yarn is reading at 21.5 stitches in 4 inches and you are a 41 inch bust and would like 4 inches of positive ease, you are looking for a stitch count around the chest to be about 242 stitches.
When you’re considering sizes in a knitting pattern, please ignore the labels we place on standard sizes. If you are usually a medium sized top from a commercial manufacturer this is not likely the case when knitting up a sweater. Depending on gauge, style and desired ease you may end up following pattern instructions for sizes labelled larger or smaller.