Center Overflow - Size question

[Der_Iron_Chef]

I'm assuming you mean in the exact centre of the tank, not the centre against the back wall. I don't think there's a certain size of tank that's necessary, but keep in mind that at the smallest, the overflow would probably be 6"x6". So I guess you need to figure out how much space you want between the overflow and the glass.

I have been considering this as well for a new project, and would likely build up some rockwork around the overflow, so I wouldn't just have a big black overflow box jutting up in the middle of the tank.