It's a 90 degree FOV, Each eye gets half of 1920, then divided by 3 (30°/90°), you'll get 1920/(3*2) = at least 320 horizontal pixels to work with. There are more pixels in the center due to how the distortion works, so in practice a little more than 320.

(edit: actually, if I recall, the 90° FOV on DK2 is actually a diagonal FOV, so you are looking at a ~78° horizontal FOV)