Half of Kurama's chakra was sealed, so theoretically he is at half power.
However, there is some evidence to suggest that Kurama could have regenerated his other half. Take for example the fact that if a bijuu dies, it just regenerates over time. Also, some people have done size comparisons with BM and Kurama before he was sealed.
Basically, it's whatever you want to believe right now because it doesn't hold that much consequence.