How big hard drive should I buy for Time Machine?

I believe TM recommend 2x to 3x the size of your drive, so I'd say 1 to 2tb drive should work

I've always just used a Time Machine the same size as my harddrive, with a couple of gigabytes to spare. You can do this if you only want to save a backup snapshot in case of your computer failing.

You can also add some more space to the Time Machine and use it to store many many versions of some files that you edit frequently -- but since Time Machine only makes multiple copies of files that you actually change, I think 2x to 3x your disk size is massive overkill, unless you need several dated copies of giga-sized files.

I do not use time machine because I prefer to have a bootable clone in case of HD failure, however if you want to use TM, I would suggest that you partition a 2TB drive into two partitions of 512GB and 1.5TB then use the 1.5TB partition for TM and the 512GB partition for a bootable clone using SuperDuper.