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It should probably be slightly larger. If you have the bulkhead you can measure the width of the flange and gasket. That's your wiggle room. The flange is the "lip" or flat disc-shaped piece that sits flush with inside of your tank wall and the gasket is the flat rubber ring that sits against the tank wall from the outside. (Someone please correct me if I have that wrong).
If you make the hole too small (as I did), you can enlarge it with a grinding stone, but it can be tedious and require different bits and more risk to your glass.
If you make it a bit too large, the flange will probably still cover it. If you make it way too large, then you're toast and you need a larger bulkhead.
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