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

As Joe417 said, there are rubber grommets that allow you to push the vent pipe down into the grommet. Only issue is you wont have a perfect watertight seal, so you don't want to fill the tank past full. The grommet does have a "stop" molded into it, but you can push past it. So you have to make sure not to push the vent pipe to far into the grommet as that wont let the tank get to full capacity. .

Grommet link