5 gallon bucket

So I am about to create new waterers for the chickens since Texas is already getting too hot. I am looking to switch from cups to nipples. I have read that the nipples can create a vacuum and cause them to not work. I have yet to see how big of hole I need to put in the top (would like it small...

We have a closed water system (as good as you can get in your backyard setting), where water is automatically pumped via outdoor faucet timer into a 5 gallon bucket (located in the coop), which feeds down to a pipe with nipples for our chickens to drink. With it being so hot out I figured I'd...