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)