I don't think it was the same size.You need both eyes yes but it doesn't change the fact his use of Susano'o was better than everyone else's. Madara displayed the same sized Susanoo as Rinnegan Sasuke and the same feats yet he didn't have Rikudou chakra.
Rikudou Sasuke's PS was literally the size of chunks of Juubidara's Meteors which dwarfed the mountains which were roughly the same size as Madara's PS. I think Rikudou Sasuke's PS was substantially larger.