High Resolution Video
The video resolution that can be played is only limited by the video decoder capabilities of the hardware. Most modern systems can decode at least 4K H.264 or HEVC(H.265) in hardware. Some systems (especially mobile / untethered) have some custom encoding requirements for higher resolutions and the developer documentation should be researched.
In general support for H.264 above 4K resolution is not universally common. For resolutions above 4K the HEVC (H.265) codec should be used unless targeting a specific platform with other capabilities.
Hardware has a limit to how much video they can decode simultaneously. It may be able to load 4 1080p videos and decode them, but only 1 4K video. This is especially true on mobile / portable VR platforms. If you use up all the capabilities of the hardware video decoder, then a second video will often not be loadable until you have unloaded the first video.
On Windows 8K video decoding usually requires a high-end GPUs (NVidia Geforce 10xx series and above, or newer Intel integrated GPUs) with 64-bit builds, using the HEVC codec.
For very high resolutions (eg 16K and beyond), a flexible codec like Hap or NotchLC can be used.
See the performance notes for the Android Platform about using OES mode for best high-resolution video performance.
The Quest has special encoding requirements which are detailed in this Blog Post.
macOS / iOS
See the performance notes for the iOS Platform about using YcbCr mode for best high-resolution video performance.