More seriously though, obviously too big depends on how much you're willing to put up with.
Once you get to a size where you can't reach anywhere near the back or the bottom of the tank the maintenance starts to get more difficult. Bigger tank=bigger and longer water changes, more cost for heating, lighting, etc. Once you hit a certain point in expenses and labor needed to maintain the tank and it stops being fun and becomes a chore. That's the point where it's too big.
I think 4X2X2 tanks are about the perfect size. Fairly big, but not too big.