The maximum size of one repository for O365 is 64 TB. That is the maximum one Jet DB can handle (which is the format where our repository is based upon). Veeam B&R itself can handle the backup of a VM with a 12 TB disk. But the problem I see is the bandwidth. Having 12 TB of data with the initial download of all mails from O365 can cause a lot of issues. MSFT has throttling on it so it will take lots of time before that is finished.
Personally, I would wait for v1.5 (beta now, GA soon) that can handle multiple repositories and multiple proxies to divide the load