I bought a 20x20" for 1000 yds plus. Seems big but I always seem to be with newer long distance shooters. It's nice to see the hit and help them adjust to center.
An example was last year when my youngest came out to shoot. He has talent but doesn't shoot enough. There are permanent 20" targets at 500, 800, and 1140 yds where we go. I had him go prone with my gun at 500 and his first shot was 8" right. Had him send another and it was within an inch of the first???? I told him to try again with wrapping his thumb and the next shot hit the 2" white bull. Being able to see the first two helped with fine tuning.