Found this, hope it helps.
If you don't have spare yarn for a swatch, then you will want to carefully control how much your item felts by checking it during the boiling process.
•Try fitting your item in a pot and find one that will comfortably contain the item you want to felt, with a little room for movement.
•Fill the pot half full with water and bring it to the boil.
•Place the item you want to felt into the boiling water. Make sure that the whole item is immersed in the water. Push item down and agitate a little if required.
•After a few minutes lift out your piece with tongs or a wooden spoon and check your item for size and finish. If it has not yet shrunk enough, place it back in the water and allow it to boil a little longer. The longer you boil the thicker the fabric becomes, also - the stitches begin to lose definition and blend together.
•When you feel the piece is the right size, turn off the stove, remove your boiled wool item from the pot and plunge it into cold water. This fast cooling fixes the wool and helps prevent further shrinkage.
•Squeeze out the excess water (don't wring), roll your item inside a clean dry towel, pull it gently into shape and allow it to dry.
Once you have your boiled wool item the size you want it - future washes should just be a regular wool wash or a rub through by hand to avoid further shrinkage.
So - you have the ability to stop the felting process when you have the size you want. You should expect your item to shrink between 25% and 35% smaller than your original pre-felted size.
For example, if you're making a bag and the pre-felted size is 12 inches - you can expect it to shrink to 8 - 9 inches. If you want your bag to be 12 inches finished, then before felting it should be 16 - 18 inches.
It all depends on the density of the yarn (how tightly it's spun), the stitch used and the hook size as to how densely your item will felt. The smaller the stitch and hook (ie. single crochet) the less the shrinkage, the longer the stitch (ie. double crochet plus) and larger the hook, the more shrinkage you can expect.
This method of producing boiled wool projects is quick and easy. Don't forget - your yarn should be at least 60% wool or animal fiber to felt well.