I've mostly designed accessories up til now, so sizing hasn't been a huge issue for me. When my patterns are available multiple sizes, it's only two or three - you don't need seven different sizes for a sock or gloves. And while I really love accessories, both designing them and knitting them, I do have a few garment designs in my head. And that's where the big G Word comes in: grading.
Grading is the method of sizing your garment into the correct range of sizes you need. It's not as simple as you think, because every size has different proportions - you can't just multiply all the stitch counts of a 24" sweater by two to get a 48" sweater! That would be much easier, but people just don't work that way. As the size of a sweater goes up, the measurements change in different ways.
So grading involves a lot of math, and a lot of sizing tables, and a lot of measuring. With all that in mind, I'm already not looking forward to it, but there's still another problem, and that is the fact that grading knitwear is something of a trade secret. There have been several threads in the Designers group on Ravelry, and the consensus is that most design books are geared toward the home 'designer' - that is, not someone who releases patterns in multiple sizes, but someone who is knitting just one sweater for themselves or a recipient. So, even the most highly regarded books on sweater design do not cover grading at all.
So where DO you find information on grading your knitwear into multiple sizes? Most people recommend books geared at fashion design students (though they are written for sewing, the information also applies to knitting). Before you get to that, however, I have found a few links to get you started.
First, there are two Knitty articles: Multisize Me, which deals with the nuts and bolts of sizing, and Multisize Me More, which talks about yardage requirements and adding stitch patterns into the mix. These articles are a great intro to grading, with the kind of information that is scarce to come across anywhere else, much less for free!
Marnie MacLean, author of numerous computer-knitting tutorials and a fabulous designer in her own right, has a tutorial on grading with Excel. She even offers the spreadsheet for anyone to use. While the two Knitty articles provide the theory, this article will show you how to put that into practice, without having to run each set of computations individually.
Finally, Ysolda's expanded sizing chart is a size chart with a knitter in mind. Ysolda's patterns are offered in a huge number of sizes, which is great for all knitters - the large and the small. She has kindly offered her measurement chart to the public, so we can use it as a reference in our own grading as well.