With Captivate 9, what is the ideal video size for desktop?

I want it to size down well, but still be big and bold on desktops, which a lot of my kids will be using, and haven't found the numbers I need for recording.

Advice, please? I'm using Screenflow, on an iMac running Yosemite.

(I know I've seen it here before, but it's not coming up in my searches. Sorry for dupe.)

Thank you so much, in advance!