How big does the waste tank vent hole need to be?

I have a '01 5th wheel and an '05 TC and both have a sliding vent. There is a rubber grommet that is all most flush with the top of the tank and is sealed into the tank with some type of putty.

The vent pipe is pushed into the rubber grommet after the tank is in place and then mounted to the campers. This allows the pipe to move up and down as the camper flexes.

I can't remember the exact size, 1.25 or 1.5 inches.