Stick with the smaller sizes. Just too great a chance that the poster board will get bent if in the overhead bins. Unless you were on a midweek, mostly empty flight, and could get a bin just for the poster board.

You can put a small poster board between the seat and the wall but that is also risky, unless the flight is mostly empty.

It seems easier to bring all of the photos with you and have someone pick up the poster board and tape at the destination. A couple of hours of creative therapy awaits the group as they create the collage before the memorial service.