800x600 is very close to my 700px limit. I doubt it would make any difference.
It's really a judgement call. It all depends on what sort of problems and unauthorized uses you want to protect against.
For example, if you have some high rez landscapes that you want to advertise, nobody in their right mind is going to try to steal your image and print knock-off 16x20's or posters from even a 1200px wide advertisement. OTOH, if you are specializing in making tiny buttons, avitars and such, and are worried about people "appropriating" them, even something as small as 200 px on a side would clearly be asking for trouble.
If you are worried about people stealing your cell phone / instagram pix, (a) will there be any economic loss to you if they do; and (b) just plaster your name and a copyright sign over the image as a watermark and, if done correctly, someone will have to go through a lot of trouble to remove it, so they will likely find lower-hanging fruit to go after.
BTW, sometimes having (a) happen isn't all that bad. If you are just starting out, having your pix spread around without you having to do any work to make this happen can be a very good thing for your reputation as a photographer / PS artist.
HTH,
Tom M