The last time this happened I contacted support and they had me increase the hard drive size of the replica vm, which didn't work.
Which VBR version are you running? Could you also porvide case ID you are reffering to? VB&R should handle resizing source replica disks by merging existing snapshots and making corresponding changes on replica VMs. If that doesn't happen, please, open a ticket with our support team and provide case number here.