Yes! Your interviewer was very correct! There is a very precise formula for sizing the earth strip:
S = I Root 't' / K
where, S is the cross sectional area of the earth strip, in sq.mm
I is the maximum anticipated earth fault current, in Amperes
t is the duration of the earth fault in seconds
K is a constant dependinng upon the material of the earth strip, type of insultaion & method of laying the earth strip (Pl. refer to IS 3043 for the values of these constants)