How to Unshrink Clothes (Wool Sweather, etc)

Here is an article which tells you exactly how to unshrink wool.

When wool gets wet and warm, the fibres in the wool lock themselves together and don't want to let go, resulting in shrinkage (you can get wool warm or wet, but not both), which kind of makes washing wool garments a dangerous business.
To un-shrink the wool, soak the garment in warm water with a mild wool friendly soap for about 10 minutes. This unlocks the fibres in the wool. Then lay the garment out on some towels in a cool place. Stretch the garment out to its original dimensions. The stretching pulls the unlocked wool fibres away from each other. Once this is done relax and allow your wool garment to dry. The absence of heat from the drying process allows the wool fibres to set in place without locking together and shrinking the garment again.