When you want to felt something, it can be difficult to estimate what the size of the finished product will be based on the item before it is washed. What I like to do is figure out from the number of stitches and the finished measurements (either those I plan to use if I am making it up, or those given in the pattern otherwise) what my finished guage will be. This is the part where you do lots of swatching. Remember that how something felts depends not only on the fiber, but on the brand, color, and your washing machine as well.
The two green swatches are actually identical-one is felted, the other not. Your swatch does not have to be big, as felting is far from exact. In this case, the standard sized business card on the right shows the scale.
With this particular project, I used sport weight wool. I like using lighter weight yarns for felting, because it keeps the finished product from being to bulky or stiff. A good guage for sport weight yarn, in general, is about six stitches per inch, and I would have gotten that guage with this yarn on a size 5 needle (US). Because it was a felting project, I wanted the finished product to measure about that guage, so I used a size 8 needle (US) to get about 4.5 stitches per inch, and felted it until it measured 5.5 stitches per inch (close enough).
While I did not try to get any specific row guage, it is also important to note that I kept track of both my before and after measurements for the row guage as well.
Using my stitch guage and row guage information, I was able to figure out how many stitches I needed to cast on, how many inches I needed to work, and how many times I needed to wash the item to get the finished product. In this particular case, I was making a hat:
The math told me to make the hat about twelve inches long before decreasing for the crown, so it looked really long before I felted it. The finished product looked like this when I put my husband under it:
A perfect fit!