It was much easier to measure DD's feet when she was sleeping, even if she was in her pajamas...Trying to do it when she's awake has been pretty amusing because her favorite thing to do is to kick her feet!
So at 6 months old, her ankles are about 5 inches around, and her feet are about 4.25 - 4.5 inches long. Of course babies come in a wide variety of sizes, but at least that should give you a ballpark idea of what you need.
Hope that helps, and sorry it took me a couple days to get back to you. What pattern are you using? If the whole sock was 7 inches long, that would probably be about right. But if just the bottom of the foot is 7 inches long, that would be a pretty big baby!