The bigger the better, but number 6 solid is kind of a stantard with antenna
suppliers that furnish it. I use whatever I have that is high current
carrying. #4 or #6 or bigger. Grounding is an art of it's own so you
should study it as any other aspect of electronics. The experts run all the
ground absorbers to a common buss bar and and the ground collectors to that
bar. Make a study of this as there is no easy answer.......Rollie