SJ - the sock is 12" diameter. I've never used it in serious current but there have been times when current in opposition to wind direction has allowed me to remain stationary over feeding fish
If the wind is light and I want to cover a lot of water I dont deploy it. Once I locate fish though, I wouldn't be without it. Being able to cast downwind and stay in contact with the fly without stripping like a maniac makes for a comfortable day.
In addition to slowing down the drift, I can also adjust the direction by altering the position with a sliding pulley system - forward/aft. I should note that my "pulley" system doesn't include any pulleys - just a loop of very stout nylon strung between a couple of eyelets at the stern and just forward of midships. About as cheap a setup as you could get - but it works.