I will go with 3/0 CU but may i ask about the service size you say 1300 amp ?? this is kinda oddball size if this was on 1200 amp service i can understand then you use the 3/0.

But for ground rod wire useally #6 will do it but becarefull here some area will required larger one typically #4 or larger

Merci, Marc

Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)