I'm working on socks, as a break from mittens, and it occurred to me that there may be knitters who haven't made socks in worsted weight, or haven't made them for an abstract foot. . . and sometimes it's hard to figure out what's too big or too small when you don't have an actual human right there with you. Of course, you can borrow a representative human of appropriate size and measure its feet.
If that's not an option, there are several charts of sock sizes out there on the web. My favorite is the Sock Size Research Project put together by Mary Moran, who is listmom of the Socknitters group on Yahoo and a former yarn store owner. Everyone was invited to submit their own measurements and those of friends and family, so each size lists a variety of measurements. You can see the range for yourself, and figure out what's reasonable (all measurements are in inches, so non-U.S. readers will have to do some math). Here's the link:
Sock Size Research Project
That link takes you to the women's page; there are links to click at the bottom for children and men -- and, if we get to knit for babies again, there's a link for them too.
For kids aged 7 to 14, any foot length (this is measuring from back of heel to tip of toe) from about 6.5 inches to 10 inches will be useful.Remember that many 14-year-olds have "adult" sized feet, so don't neglect the larger sizes!