Get a soap that is meant for wools...my favorite is Eucalan and a knitter friend also likes Soak. (Nothing against Woolite, but I was told when I started knitting to avoid it so I have.) With Eucalan, you do not have to rinse the garments. It also comes in a variety of scents, including NO scent, which is great if you have a sensitive nose.
Submerge the sweater in a sink full of water, not hot or cold, and a cap of Eucalan for 10-20 minutes.
Drain the water and then roll the sweater in towels to absorb some of the water. I usually roll them in 2-3 different towels until the sweater isn't wildly dripping any longer. (For the demo photos I needed to block one of my baby sacks, but the concept is the same for all wool garments.)
With your wet sweater (and wet wool is stinky, btw), you now want to place your sweater in a place were you can spread it out to the size you want. I have a nifty blocking board with measurements on it and T pins. However, you can put down towels on your floor and put the sweater on it, too. I would recommend that you invest a few bucks in some non-rusting pins. Pin one side of your sweater down and then stretch it to the desired width and length, pinning it down as you go. Merino wool will typically let you adjust it about 2 inches. You do have to go through this process each time you wash your sweater, but it really helps with sizing and to lengthen the life of your garment.
In addition to cleaning your garments and getting the size you need, the other positives are that it softens the wool and evens out the stitches (i.e. makes them blossom).