The vast majority of bluegills that I catch that are over 10" long go back in the pond. This has increased the size and number of larger fish in all the ponds that I fish.
Not real scientific, but it has worked for me. Also leaveall the bass so that may help alot. But when ponds with 6" gills go to gils that are 9" and better, I keep thowing the largest ones back in.