It’s not in the plans because people use different tap towers that might require a different size hole.
But, in most cases it’s a 2″ hole.
Also, remember, you should insert a 2 foot (or shorter) tube extending from the inner top of the cold box up through the tower, then run your beer line through the PVC tube.
Just use the white schedule 40 1.5″ ID stuff you find at Home Depot.
This allows cold air to circulate up into the tower, keeping your beer lines cold all the way to your glass.