A zig-zag transformer derives a neutral from 3 'hot' phases. This neutral can then be grounded.
A stand-alone set of wye coils doesn't do a good job of deriving the neutral, because the current flow through a single wye coil is limited by its inductance; it is like the primary of a transformer with no secondary load.
You _can_ use a wye-delta transformer to derive a neutral. The wye is connected to the supply and the wye neutral is your derived neutral. But if any current flows to the derived neutral it will induce current to circulate on the delta, avoiding the current limiting of the stand-alone wye.
As far as how to size a grounding transformer, the question is: how much current is expected to flow, and for what duration.
If you want to solidly ground your system the way an ordinary wye secondary is grounded, then you need to size the zig-zag transformer to carry the expected fault current for sufficient time to trip OCPD.
If you instead use a high resistance ground system, which gives continuity of service benefits combined with grounding benefits, then you need to size the zig-zag transformer to carry the expected _continuous_ fault current through the grounding resistance.
-Jon
