Neutral size in breaker box

You can always make wires larger than are strictly required.

So if your hots and neutral all must be #6, you are welcome to make the neutral #4.

In fact, one sneaky reason to do this is that a #6 neutral MUST be white or gray. Whereas a #4 can be generic black, and marked with white or gray tape to designate it as neutral. So if you wanted to avoid a run to the store, it might be a good play.

The Code requirement there is that #6 or smaller wires must be white or gray for neutral, green, yellow-green or bare for ground, and hot can be any other color. With #4 or larger, the electrical supply houses don't want to stock 3 colors of the big stuff, so Code allows you to re-mark with tape or paint to any base color, to indicate the function.