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.
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~Drew
10G Nano * 10G Sump * Deltec MCE 600 Skimmer * JBL Viper 150w MH * Zeovit * Vortech MP40W
Millions long for immortality who do not know what to do with themselves on a rainy Sunday afternoon. ~S. Ertz
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