How To Male Cisterns

Our friend in the illustration has a large sheet of zinc, measuring (before cutting) eight feet by three feet, and he has cut out square pieces (all of the same size) from the four corners and now proposes to fold up the sides, solder the edges, and make a cistern. 
But the point that puzzles him is this: Has he cut out those square pieces of the correct size in order that the cistern may hold the greatest possible quantity of water? 
You see, if you cut them very small you get a very shallow cistern; if you cut them large you get a tall and slender one. 
It is all a question of finding a way of cutting put these four square pieces exactly the right size. 

How do you avoid making them too small or too large?

See answer

Math Genius