It depends on how much fault current u would like to allow to flow through stator winding to core.l.e earth when fault happens.usually it will be coordinated with earth fault limit or damage curve of generator this will be set
..if u would.like.to limit 5A in 11kv system…directly take 11kv/root3 =phase voltage /5A will give u the resistance value..this is high resistance grounding.we can go for low resistance grounding also.if u want you to be selective protection
Main aim is not to oversize.and not to cross certain level of temp leading to failure..before that time the system will be disconnected incase of any fault
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