Neutral Size Compared to Phases

Yes. It depends on the possibilities of phase imbalance and presence of harmonics, for instance.

I agree with T and T. if you have a high harmonic load, the neutral may be larger than the phase conductors. The other factor is how well the load is balanced.

Check these sites for additional information

http://www.electrical-installation.org/enwiki/Sizing_the_neutral_conductor

http://www.tlc-direct.co.uk/Book/4.3.12.htm

Not "ALWAYS". It depends largely on the distribution and/or utilisation of single-phase loads distributed on to three phases. Under the worst conditions, it could just be possible that only one of the three phases could be loaded to full capacity with the other two phases unloade, in which case, the neutral MUST be sized equal to the phase.

And.... even that is not enough today. Today, with the advent of power electronics, one needs to even 'double-size' the neutral (i.e.) the size (cross sectiona area) of the neutral conductor is twice that of the phase conductor, thanks to triplen harmonics.