FYI, for anyone else going this weekend and hoping to see the show, some of the signs that pointed to it were the obvious stanchions going up all around the Tri-Wizard stage and opposite Filch's. Team members were told it was for a "special event" and provided no other details, but when we asked people at FotH, they flat out said it was for the light show at 8:45 and 9pm (though those times were a tad off, it was more like 8:40 and 8:55... but that may also be because of technical previews being subject to change). The other big sign: all the Frog Choir and Tri Wizard shows were done by 6:00pm, even though the park was open till 9pm. Check the mobile app to see.
Also, if there are two shows, the area totally emptied out after the first show. They sort of tried to kick everyone out, but then let people back in, as they seemed to be getting a count of how many people were in the viewing area (they were using a clicker). In any case, the viewing area was virtually empty for the second show. We stayed by the fire hydrant to the left of the Tri-Wizard stage, and it seemed like that would be the best view.
It's really hard to say how many people the area holds. It doesn't seem that large for a show, but it did seem to hold quite a few people. For obvious reasons, the first show of the night was the most crowded.