What I would do is set up the pj on a table, and project onto the wall, and see how big the screen would go. Try various sizes of projected image. Sitting closer thatn 2 x screen width may start to reveal screendoor (gaps betwen pixels), so the above method can help in two ways.
Watch a few movies and see which size looks good, then buy a screen to match the size you liked.
EDIT:
A quick look at the throw chart would put an 80inch wide screen roughly in the middle of the min/max zoom, so provided you're sitting 13.5 ft or more away, that size might suit you.
Gary.