There are three standard Facebook sizes. One is 720, as you mentioned, but it's very small and is rarely used these days.
The next is 960. It's also smallish, but good and fast to load.
The third is 2048. This will be sure to fill everyone's screen. But of course is the slowest to upload, and load for view. Still, it's the most common one that people use nowadays, I think.
Which do you reckon you'll choose?