In my opinion, no matter what size hard drive you get, it will never be enough for your work. Relying only on Time Machine doesn't cut it for me, because it would end up deleting the old backups to free up space for new ones. So plan on using multiple drives from the beginning. This is how I do it (this is not optimal, and I am looking for alternative ways so I will be monitoring this thread closely):
I download from camera to my rMBP and usually edit on the internal SSD. Then I move the photos (using Lightroom catalog) to a 2.5" USB3 drive where I can continue editing and store the photos. I always have a size-wise matching 3.5" drive for a backup, and I also use a simple dock where I can just drop the 3.5" drive and start copying. When my USB3 drive is full, I buy a new one and a matching 3.5" drive. The photos are chronologically stored in quite a few Usb3 drives and there's always a matching 3.5" drive for every one of them.
Currently I'm making the backups manually with Beyond Compare, although there must be easier ways. However, it's not too difficult, since you just have to sync from the usb3 to the 3.5" once in a while. If the usb3 fails between my backup syncs, I can be sure that there's at least a few months' photos backed up by Time Machine along with the usual stuff inside my Mac. That's because I originally downloaded from the camera to the internal SSD, which is being time machined. Dodged that bullet.
I tend not to go for the biggest drives, because you can fit a lot of photos in 1 TB and the smaller the drive, the less you lose in case of a total disaster. If I need to go back a few years, I just, well, go back a few drives.
If you decide to do something like this, remember to check that all of the usb3 and 3.5" drives are working once in every few months. And also, think about the file format. I'm using dng with embedded raw and jpeg, and I'm hoping that it will be readable long after Adobe or any other company has been bought, turned around and sold twice over.