File Type: JPEG
Color Space: sRGB
Resolution is irrelevant as the file will be identicial no matter what number is used (unlike a print)
The rest depends upon you.
Size: depends upon how big you want it to look, 800px is a reasonable size that is big enough to see but small enough not to make high
Quality: Mid-range to balance between speed and quality
Here is an example screen showing Export settings that will work and can be tweaked to your desire. But in the end you have to decide some things on your own.
I use resolution 72 which is really ugly if printed... I think watermark is distracting to the viewer
Also, the Kodak "golden standard" 778x517 is often used for this purpose. While it's pretty close to 800x600, supposedly it displays a bit faster, especially true for slower systems
As Rikk previously mentioned, the file export specification of "resolution" whatever the number (even 72) has no bearing on someone's ability to make a print from the file at whatever resolution they choose (within the limits of the pixel dimensions you make available); it's the pixel dimensions that will limit printed size/quality. Perhaps you meant that you use a jpeg "quality" setting of 72, which is indeed fairly low. That might even be set even lower to enable reasonably good web display yet discourage printing.
As to use of watermarks, yes, I agree they can be distracting, but not necessarily. A watermark can be chosen that is a compromise--minimally distracting yet prominent enough to discourage some (maybe not many?) from making a print.
As to the supposed (Kodak) "golden standard 778x517" : I wonder if you are thinking of 768 x 512? In any case, it's certainly not in any way a unique dimension that offers a special advantage in display speed. Sure it should load "a bit" faster than 800x600 since it contains a smaller number of pixels.