Technically, you only need as much space as you are trying to back up or as much as your main drive can hold. Your system has caches worth gigabytes and other things that are never backed up. Thus a 2 TB drive would be enough to make a backup for a 2 TB system. The only reason why you should use more is to make use of snapshots. Depending on your disk usage, filling up the remaining space can take weeks or months or even just an hour if you make very big changes.
I only use a ratio of 1:1.2 of my drive’s size, as my main disk is never filled anyway and I don’t really need the snapshots, aside from a buffer for system upgrades. I still have two months worth of snapshots.