Well, among things like a tapered multi-bit that will drill out the holes to a larger size, there is another method that has a larger variety of knockout sizes. Knockout punches incorporate a set of die and knockout pieces, along with some sort of device to pull the two pieces together to punch a hole in the box.
One such thing is a screw-type, manual knockout that ca be tightened with a wrench to knock out the hole. As the wrench is used to tighten the bolt head, the two pieces are drawn together on either side of the box hole opening. Continue tightening until the knockout is pressed through the metal of the box opening.
The other choice is a more expensive knockout set. This one is a hydraulic knockout punch that uses the same die and knockouts, but incorporates a hydraulic cylinder to drive the knockout through the box. This too can be a manual, hand-driven pump, or you can purchase a more costly electric hydraulic pump that works with the push of a button.
Having a set of knockout punches can save you when you're in a pinch without the right sized opening in a box or electrical panel. I personally have a screw-type, manual set that is just perfect for that occasional knockout need. If you frequently do electrical projects and have run into these difficulties, a knockout punch may just be for you.