I wear both 5″ and 7″ cycling socks but prefer the 7″. I’m not sure why. My calves are skinny I’m told, so they stay up easily. I prefer to think of my calves as lean and strong, like those that only long miles in the saddle produce.
And, I prefer wool, even in the summer. They are never too hot. And, they keep the feet warm even when wet. They also seem to provide more cushioning for the balls of my feet on long rides.
I wear 7″ wool socks all year.