What is the maximum 2D video quality on Oculus Quest? If you want to create a 2D video to display on an Oculus Quest VR Headset, what is the highest video resolution the Oculus Quest is capable of displaying?  

720p or 1080p or 4K or what?  Do the technical specifications shed any light on this?
2560x2560 per eye 

The maximum recommended video resolutions for Oculus Quest and Go are as follows:

3840x3840 30fps H264 H265

4096x4096 30fps ---- H265

5760x2880 30fps H264 ----

2880x5760 30fps ---- H265 (* see below)

2880x2880 60fps H264 H265

4096x2048 60fps H264 H265

taken from here