60 pixels per degree, 210° by 100° for full 20/20 vision (most people have slightly better)[0]. Headsets right now are ~12 pixels per degree.

Thats a 12600x6000 screen = 76megapixels. 4k is 8.3 megapixels, 8k is 33.2 megapixels. 8k/eye sounds about right.

[0] https://youtu.be/Qwh1LBzz3AU?t=23m39s