Bob:
Howdy, how's it goin'?
Size I have been pricing is the common 3' X 5' size.
One web site even has suggested flag sizes for different flagpole heights, and that's what is recommended for flying at a height of about 16 to 20 feet or so, which I assume is what most clubs' poles would likely run .... either that, or around a 4' X 6' size, which I think is a bit too big, especially since the cost would go up quite a bit.