All of our demo sites now use the full screen option, using the 1000px images.
On certain very large monitors, for certain images, you can see a slight image quality dropoff if you really examine them but overall they look good and some images look great. You could upload larger images for the full screen galleries, but with any more than 5 or so images, the page will start to load slower, especially on iPads or mobile devices.
We purposely left the images in demo sites at 1000ps wide so it would be a realistic look at the galleries. We also saved these images using Photoshop's Save For Web as jpg's using anywhere from 60-80% as the quality setting. So there is lots of room to play with them and possibly improve image quality even more – but this improvement would only be visible on monitors over say 23 inches anyway.
If you are happy with the quality of the demo sites, then 1000px wide, saved at 80% is enough to have great images and keep a good balance with page load speed. The other benefit of 1000px is that all the images will work in any version of the galleries as well in all our slideshows and in the blog posts. Simply for ease of use and streamlining work flow, we have been recommending that everyone just use 1000px for everything.