There comes a time in every knitter's life when she or he wants to make a project but finds it is completely the wrong size for them. It's important to be able to make pattern modifications yourself so that you can have garments that fit perfectly.
Using the Convertible Knitted Bodywarmer as an example since the sizing is very easy, let's go through the process that was used to design the pattern so you know how to fit yourself if you want a larger or smaller size.
I know people generally don't like to make swatches. I was the same way until I started designing. A swatch is the only way to know what the yarn is going to do before you start working on your project. Ultimately it saves you a lot of time if you judged incorrectly and have to rip out your work and start over.
With your selected yarn or yarns and a size 15 needle, cast on 12 stitches. Knit 12 rows, or until the swatch is about square. The number of stitches per inch is more important than rows per inch on this project. Bind off.
Stretch your swatch only as much as is needed to make the sides straight. Measure the width from the cast-on side. I got four and a half inches.
The Math Part
The math required for this pattern is really easy. First, you need to figure out how many stitches fit in an inch by dividing your width into your number of stitches. In my example, that's 12 divided by 4.5, which gives you 2.6.
Then you need to measure how big around you'd like the finished project to be. Use a piece of yarn or a flexible measuring tape to circle your arms just below the shoulders. Shrug or stretch a little bit so there's a little give built in. I got 43 inches, then I added an inch just for extra flexibility. You can do that or not.
Then multiply your number of stitches per inch (2.6) by the number of inches around (44), which got me 114.4.
Now comes the only tricky part. The pattern used is double rib, which requires a multiple of four stitches. So instead of 114 stitches, we need 116 stitches. Because we're knitting this in the round, that will allow you to knit straight, K2, P2 all the way around the project.
Not every project you want to alter will be this easy, of course. Often you have to deal with shaping and basically redrawing a pattern to get it to work for you. But this process works for things with minimal shaping like ponchos and hats made with a pattern stitch that is easy to scale up or down.
Resizing simple garments like this will give you the confidence to tackle bigger projects later.