If it is going with boots: knee high or over the knee. A crew sock disappears into the boot. A knee high shows between the boot and the hem, which is the whole point of wearing it.
If it is a gift: pick the print for the person, not the length. Nobody has ever been disappointed by the length of a sock. Getting the theme right is the entire gift.
On sizing: one size fits most, except over the knee. Shoe size rarely matters here. Calf width does, and it is the only thing that decides whether an over the knee pair stays up or spends the night at your ankles.
If you want them to last: knit-in beats printed. A pattern made in the yarn holds its color. A printed one fades, fastest on a white ground. Inside out and cold either way.
If it is for photographs: length over print. A knee high changes the shape of the whole leg from across a field. A crew sock, however good the design, reads as a sock.
On building the rest of it: legs get styled as one piece. Socks, stockings and what you wear over them are one decision. See festival and rave for the outfit they finish.