Neutral earth flat requirement power ratings in step down transformer

i need know how much of neutral earth flat required for different power ratings in distribution transformer. in earlier we have 500Kva transformer with neutral earth flat of 25 x 5mm (cu) later it was replaced by 950KVA t/f and with neutral earth flat of 50x6mm (cu) on what basis did they change those earth flat is there any formula or any standard specifications.