My position on that is clear: no watermarks.
Just look at this site owner's. No watermark, no copyrights in the image. Nothing but the pic. Just the right combination between good size and not usable size.
Generally 700-800 as max.
When I check from time to time Magnum website, I do not miss the size. Pics are talking by themselves even at a small size.
I've been watching many top works displayed at 500 pixels. No problem.
Many pro photographers members of this wesite have their pics with no watermarks even big sized. And they are pro studios.
Just check, you'll see.
Example of a LU-LA's member: http://www.adammork.dk/
I've been impressed by his work, his website is well implemented IMO.
No watermark neither copyrights on the images.