Obviously, the larger fish give larer fillets. However, large bluegill are an ever increasing rareity and worth protecting IMHO. 6" fish are ahppy medium and I would suggest that killing any gill over 8" is a crying shame. On teh occaion I do kill bluegills, I opt to kill a few more smaller fish and let the big girls go to spawn and fight again.
I know of one pond that grows them regularly in the 9-11" range and can tell you it's a rare resource worth protecting!