This is an interesting question that nobody has the answer to.
Chakra monsters like nagato, naruto (without kyuubi), hashirama are all part of the sage's linage and their chakra reserves i believe pale in comparison with the sages.
Secondly his accomplishments hints that his chakra levels is most definitely bijuu rank, if people like kisame and A's chakra reserves have been given this title, the sage's chakra reserves topples them.
The 3rd raikage a chakra monster collapsed with exhaustion i believe after his draw with the 8 tails. The sage, it was never hinted suffer this chakra depletion when he defeated and sealed the 10 tails which is made up of all 1,2,3,4,5,6,7,8,9 tailed beasts.
Secondly the sage used banbutsu sozo to split the 10 tails into 9 parts and sealed its body on the moon with ct. Nagato stated this his ct was piss poor in relation to the sage and he was right. His was big, but not as big as the moon.
Thirdly in order to contain something like the 10 tails chakra, you must have a stupid level of chakra or else such foul chakra will easily push your little chakra to one side and take control over you.
I do believe the sage definitely had bijuu level chakra, definately not on the 10 tails level, but a portion of the 10 tails. Yes i believe the sage's chakra is more or less equal to the fox