Most sweater patterns come with a series of sizes on the first page. They range from XS-XL and/or list chest measurements 32 and up.
For at least the last 20 years I've considered myself to be a "medium" on top and my chest size has remained a steady 36.5". Thus, when I look at a sweater pattern I automatically choose the one that says, "M" and/or to fit 36" bust.
My handknit sweaters are ALWAYS too big and that's before we even get into the issue of yarn growing. I've never been able to figure this out.
Then I went to Amy Herzog's "Fit to Flatter" class last fall and I discovered that these "standard" sizes were not what I should be using to choose MY sweater size.
Amy had us all flip our pattern pages until we found something called the "schematic". It looks something like this one
These will be the pattern's "finished" measurements. You want to look at these in comparison to your own measurements.
Don't guess at your measurements, you need to know them. Amy has a wonderful tutorial here on what measurements you need and how to take them.
The MOST helpful measurement is what Amy calls your "Torso circumference". To take this measurement, wrap the tape measure around your torso just under your arm pits (but above your bust line). My Torso is 35.75 (just a bit smaller than my bust size).
Amy told us to pretend this was our full bust for the purpose of choosing our size. In many cases it will make a huge difference.
You also want to consider "ease" -- a knitted garment is supposed to stretch over your body and it can only do that if the sweater is in fact a little smaller than you are. However, some fabrics you knit will have drape and you'll want them to float away from the body so you might choose to build in some positive ease (adding 1-2" wider than your own measurement). And of course if you are working with a stretchy plant fiber that is going to grow, negative ease will be a necessity.
Another option for sizing is to compare the schematic measurements to a similar knitted garment in your wardrobe that fits you well. This may give you more confidence in knitting a sweater that's narrower than you are.
It is possible that once you compare your measurments (or the measurements of your favorite store bought sweater) against a pattern's schematic you will find that you need to make modifications to the pattern to get it to fit you correctly.
You may need a longer or shorter arm, you may need a narrower sleeve, you might need to insert some short-rows to accomodate a full bust or fuller tummy area. You will want to go through the written pattern and insert these changes where necessary.
It all seems a bit daunting, doesn't it? But it really isn't. Most important thing here, dear knitter, is to really know thy measurements and to choose your size accordingly!