It depends if it is a portfolio showing off a few of your best images (in which case I'd not water mark on the basis the pictures should look as good as possible) or if it is a lot of photos from an event for example (in which case I would watermark).
I'd say nothing larger than would comfortably fit inside a browser window sized at 1024 x 768 and a watermark on the image somewhere. Something subtle enough to go unnoticed but recognisable if you're looking for it.
Admittedly plenty of people now have large monitors, but massive images take ages to download and anything bigger than 800 x 600 could potentially be printed unless protected by a whopping great watermark, which then ruins the image.