Tip: I would think it would be 9" and if so, be sure
to knit foot for 7" then 2" would be toe shaping. I measure from center of heel towards toe less 2" on any sock
for correct size. If you are knitting 10" sock then knit
for 8" leaving 2" shaping for toe. 2" for toe shaping
on any size sock.
Good luck.

Stephanie Pearl McPhee says in her book "Knitting Rules:"

"The length of a person's foot is about 15% of his height. If you know how tall someone is...you can probably make that person a pair of socks that fits reasonably well."

She also says that the length of a person's foot measures pretty much the same as that person's forearm, measured from elbow to wristbone. If you can measure him, he'll think you are going to make him a sweater.

Don't know if there is any sense to her theories, but there they are.

My sons wear this size, and I wear a woman's size 9. When I buy them slippers, I just try them on to see if they fit me enough. Their feet may be a tad wider, but this works for me. What size do you wear?

An idea: Measure your foot and what size shoe you wear
and maybe you can figure it out!
Wishing you good luck.

I'm going to have to go with sheilajoyce's advice on this one, as I, too, wear a size 9. These are a Christmas gift for my dentist, so there's no measuring anything! I KNOW most probably don't knit for their dentist, but I have one that heard I'd had a heart attack and called my home to see how I was doing...needless to say, this merits socks for behavior above and beyond!!!

The foot measurement of a man's size 9 1/2 shoe is 10 1/2 inches. A 9 is just over 10 inches.