That is no answer question, but the first post by electricpete sums it up. It all depends on the load!!!!
The best answer for the NEC is Art.210.19(A)(1) for branch circuits and Art.215.2(A)(1) for feeders. I had to oversize neutrals many times because of nonlinear loads and harmonics. There is special transformers called "K Type" that have up to 200% larger neutral windings.
Hope this helps,
Dave
