What are the recommended photo sizes for full-screen templates?

For templates the simple advice for the image size is — a) the bigger the better, b) stick with wider than taller (landscape) aspect ratios.

MyVR adapts the image resolution to the size of the monitor/screen/view-port of the person viewing. Since some monitors can be 21"+, the higher resolution the photos, the better quality website. Even if users upload massive photos, MyVR automatically generates photos in a number of smaller sizes so that the website chooses the best fit for any particular screen which optimizes load times.