yep, here we go again. We need to start a good, clean discussion thread that we can run for a while, close and then sticky.
here's my opinion:
ideally....
width: 1x the length of the fish (at least)
length: 3x the length of the fish (at least)
depth: 5x the depth of the fish (at least) - this is a guesstimate...use your judgement
I would at least make the depth the same dimension as the width.
so, assuming an "average" full adult fish of 30" (which it takes a long time to get there), tank should be at least 90x30x30, (AT LEAST a 300 wide, which is only 24" high). yes, there will be silvers kept in 125's for life, but that's just cruel. You're already stuffing them in a box.