THIS NIGHT has 3 sizes: S/M, M/L, and XL
S/M socks (on the left foot below) fit women's shoe sizes 4 to 10½ and men's shoe sizes 5 to 9 and are what is considered a standard "crew length" height. They really do fit both ends of the spectrum, so don't worry if you have small feet (the heel won't go up!) or larger feet (my mom is a 10½, and she can wear the S/M comfortably!). I am a 9, and I wear the S/M regularly (unless I am wearing cropped pants and want extra leg coverage). I generally advise people who wear up to a women's size 10½ to buy the S/M size unless they want taller socks.
M/L socks (on the right foot below) fit men's shoe sizes 7 to 13 and women's shoe sizes 8½ to 14. The foot part is a little bigger, the band is slightly wider, and the socks are about 2 inches higher than the S/M. The M/L are considered a standard "mid-calf" height. You can see from the photo below that both sizes provide plenty of leg coverage, but you get extra height with the M/L. I recommend the M/L size for people who wear men's shoe sizes 7 to 13 and those who wear women's shoe sizes 11 and up (unless you want more leg coverage and are at least a women's size 8½).
If you would like lower socks, please take a look at the Ankle Sock offerings (available in both S/M and M/L). These are cuffless and have enough sock so that they show and won't slip down into your shoe but are a nice option for hot weather or if you have sensitive ankles.
If you'd like taller socks, THIS NIGHT makes those, too: S/M Knee/Boot Socks (a true knee sock –– this will go right under your knee) and M/L Knee/Over-the-Calf (as they are called in the world of hosiery) Socks.
And finally, if you wear a men's 14 to 18 shoe size, by popular demand, THIS NIGHT now makes XL socks, too. The height is the same as the M/L, but the foot part is bigger.
Please email me if you have any questions!