I'm knitting sock for a size 8 women's shoe. Now I've only knitted socks for myself and I have big feet (size 10). So my question is for a size 8 do I cast on for the smaller size or the larger size?
Well, if its the same pattern and she has smaller feet then a smaller size.
Here's a sock measurement chart based on shoe sizes. It also shows detailed pictures of how and where to measure for a customized sock.