What is the maximum 2D video quality on Oculus Quest?
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