No, you don't need to change the needle size. Just when you get to the foot part after you turn the heel, it will say to knit for 8.5 inches, then start decreasing for the toe. You should knit for 9 inches, then start decreasing for the toe. The toe part is usually about 2 inches, so the 9 + 2 = your 11 inch foot. I am assuming your sock pattern is top down (top of the ankle, down the leg and then the foot to the toe).
Here is a really good sock tutorial, if you need it: http://www.cometosilver.com/socks/