My capacity is 250.79 GB and am currently using half that amount. My son says I need 1TB and suggests San Disk Extreme Portabl USB 3.1 Type C External SSD. I looked it up and it didn't mention Catalina OS but then I looked up reviews and found some that said it worked. Any opinions?
If you are only doing a clone backup, you only need a backup drive equal to the total capacity of your internal hard drive. In this case a 250GB hard drive would be sufficient, more wouldn't hurt but would be superfluous.
A "portable" hard drive, as I've mentioned, I think, earlier in this very same thread, isn't ideal, as portable drives have compromises that give up reliability for their small size. Unless you really need that small form factor, a desktop hard drive would be preferable.
I have no comments about San Disk drives. Except that I don't rate them as highly as the two brands that I suggested previously (OWC if you want something reasonably priced, and Glyph if you want a superbly reliable drive).
There is no need to look up whether any drive is compatible with Catalina. Hard drives don't care which version of the Mac OS you are running.
It's rare to find any hard drive that isn't compatible with a Macintosh, though some come pre-formatted for Windows and need to be re-formatted (by you when you receive them) for the Macintosh. I prefer to go with drives from companies that cater to the Macintosh market, which usually means that their drives are plug-and-play and that the company will understand the Mac if you have any technical problems and need help from their support department.