The earthing schemes and sizing differ according to the earthing scheme (TT,TN-C,TN-S,IT...etc). According to the required earthing scheme, you should size your earthing and neutral conductors. IEC standard 60364-5-54 illustrates how to size a protective earthing (PE) and neutral conductors. This can be illustrated in Schneider Electric's "Electrical Installation Guide" which is based on IEC 60364. Please refer to the attached document in pages 251 (G37) and 255 (G41) where it is illustrated that the earthing and neutral conductors are sized according to their respective phase conductors. There is also another method for calculating the size of the protective earthing conductor as illustrated on the top of page 252 (G38). So check your phase conductors and earthing scheme, then size the protective earthing and neutral conductors accordingly.

The choice of the earthing cable depends as well on the amount of short circuit that will pass through the cable in the event of a phase-to-earth fault. So as for your other question, and for cable sizing in general, our advice is to use Ecodial software. This software is a free software from Schneider Electric used to calculate and size electrical installations. You could implement the entire network you have on Ecodial and accordingly have very accurate sizing of all phases, neutral and earthing cables. The latest version from Ecodial can be downloaded from our website by searching for "Ecodial Advanced Calculation" in the search bar of the website or through the following link:
https://hto.power.schneider-electric.com/ecodialadvancecalculation/

Released for:Schneider Electric Nigeria