You will be able to control the fulling and stop when they are the size you want. I boiled additional water to keep it very hot. Be sure and wear rubber gloves so you don't burn your hands. Put the socks into hot water, letting them soak for about 10 minutes. Rub more soap into them and scrub them vigorously on the washboard, dipping them into the hot water to keep them hot and making sure your scrub all sections of the sock.
Primal scream is optional.
Wring out excess water and immediately plunge them into the cold water. Squeeze water through and let rest a few minutes. Take out and wring out excess water. It will take 3 or 4 of these actions before you notice a drastic difference. Keep looking at them and stop when they are the size you want.