Here's the screencap from TOS: http://tos.trekcore.com/hd/albums/2x...bleshd0104.jpg
As I do see it, the only characteristics that can be helpful to determine the real size are
a) the height of the deck levels suggested by the windows (if you disregard these huge vertical starbase skyscraper panoramic windows, the makers of TOS-R completely failed to notice: http://tos.trekcore.com/hd/albums/1x...rtialhd004.jpg
b) the size of the hangar bay door (I'm not aware they had energetic atmosphere screens during the time of TOS)
c) the size of the RCS / impulse engine block of the station, attached to the main section (if you believe that to be).
Was K-7 supposed to be a city in space (like the movies' Earth Spacedock) or merely a little outpost far out in the Frontier?
I tend to regard it rather as a little outpost, but that's of course entirely my personal take.