What resolution for rectilinear video?
1920x1080 is plenty of resolution for rectilinear videos in today’s VR headsets. Traditional video playback in VR is almost always done on a virtual screen in an immersive environment. A 1920x1080 video is enough to saturate Meta Quest 2’s display of a virtual screen that is more than 90 degrees wide.
A large movie theater screen is often around 60-72 degrees field of view, so a reasonable target resolution could be even lower than 1080p. To do your own calculation, you can use Meta Quest 2’s display density, which is approximately 20 pixels per degree (PPD). A 72-degree FOV would require 72 * 20 = 1440 pixels.