I was paraphrasing this from memory. The dependency is quite complex, it depends on the amount of degrees the text occupies in your field of vision, but also on the mode of reading you're in (very focused reading seems to narrow the fixation span). Also, please note that your eye makes constant micro-saccades (https://en.wikipedia.org/wiki/Microsaccade), and that the authors of the paper were probably compensating for these saccades as well, more or less.

Quote from the actual paper

    This experiment has provided data which begin to
    answer the question about the size of the perceptual
    span during a fixation in reading. Although it may be
    possible in tasks other than reading for subjects to
    identify letters. word shapes, and word-length
    patterns some distance into the peripheral areas, in
    fluent reading this information appears to be obtained
    and used from a relatively narrow region. Thus. a
    theory of fluent reading need not suppose that
    word-shape and specific letter information is obtained
    from a region occupied by more than about three or
    four words during a fixation. and perhaps not that
    large if the span is not symmetrical around the point
    of central vision, a question not tested in the present
    study. Thus. it does not appear to be true that entire
    sentences are seen during a fixation; in fact. for most
    fixations. not even a complete phrase will lie within
    this area.

So the "window" or "span" that needed to be un-masked was about 3-4 words wide (interestingly, this did not necessarily depend fully on the length of the words)

Yes but you're also able to focus on the words made up entirely of 'x'. I don't think you can really judge the effect because 1) it's not the same effect here and 2) you already know about it now.