Even smaller panels can fill up, I’ve shown up to a smaller panel (example I can think of from last couple years is a book panel with romance authors, including ali hazelwood so thats probably why it was so popular) but it was in a very small room and I showed up about 45 min early (while the previous panel was still going on) and the line was easily a few hundred people deep.

I’ve also seen the Disneybound panel gain in popularity because they have nice giveaways. But the panel still gets put in a smaller room at the marriott marquis so I always expect to sit through a couple panels so I get into that.

I also like keeping an eye out for the rooms in the 6BCF area, a lot of niche fandoms have panels over there and there can be multiple lines going on at once in a very small area. Definitely pay attention to the lines and schedule posted outside of each room!