If you kept reading to understand the context of that image, you'd realize that it could be any size, but that you need to enable "partial virtual environment." That was the entire point of that chapter/image, to showcase the three different modes (full virtual, partial, and full pass-through with collision).
I'm no fan of the AVP, but it is inane to post a comment on a picture from the article without taking the time to read the text surrounding it to understand the context. The blog went to great pains to set out the pros/cons, limits/advantages, just to have people half-read it or just look at the pictures...
Sure you can. Just increase the size of a virtual window to fill the entire avaiable field of view (110 degrees or so). It's basically the same effect as sitting in a close-to-the-front row of a theater.
There are VR headsets with wider FoV, but they're pretty bulky as of yet because of the limits of current costs-less-than-literally-$10,000-a-unit optics.
I'm no fan of the AVP, but it is inane to post a comment on a picture from the article without taking the time to read the text surrounding it to understand the context. The blog went to great pains to set out the pros/cons, limits/advantages, just to have people half-read it or just look at the pictures...