Bus Bar Size

Yes. As a Rule of Thumb, N should be at least 50% of Phase conductor (cable/ Bus bar) size.

Neutral bus bar should be same size as the phase bus bar. If you suspect there will be non-linear loads (i.e, harmonics) the neutral should be sized to 2x the size of the phase bus bar.

Depends first 3 phase or single phase. if single than N and P should be sized similar as what goes in from 1 end comes out from the other.

But if you have a 3 phase power supply you must consider the type of your load. if you have 3 phase balanced loads (same current in all phases) then no current will pass through the neutral.

If your load is single phase it is most probably unbalanced and some current may pass through the neutral.

So to be on the safe side usually the neutral is chosen one size less than the phase. or equal if your load is unbalanced at all

Are we sure the question isn't ground bus vs phase bus?

Ground bus work is usually much lighter, but is not intended to take any current except momentary faults.

Details, details, details.

3 phase neutral grounded? 3 phase delta? single phase ? Many questions to be answered.