The latest version of our default out of the box CMS setup has 3 sizes: 1600, 1280 and 1024. Members can choose which size to view and also download a Zip of the photo set in all 3 sizes.
I've seen some of our Erotic/Glam site customers offer 2000+ or bigger and many will offer a super high-res option for stills, especially for the Zips as Elli already said. 1024 is still "safe" for viewing the galleries online but it's a good idea to offer at least 1 bigger size.
It also depends on the niche of your site. Members of a reality or video focused site are generally less demanding than members of a site where image size and high image quality are of more importance.
