They can be a nuisance, if the polyps are touching coral directly they will sting it just the same as any hydroid, or anemone, or another coral. But otherwise, they don't actively seek to harm anything in thw tank, so unlike Aiptasia you won't have to worry about the polyps actively tring to kill any critter that comes within striking distance of them.
Larger Upside Down Jellyfish can get to around 1ft in diameter, but I think average size is about 5 to 6 inches.
Unfortunately my little Jelly died after two months. Also, the past January I was forced to move everything into a new tank, and I think the move led to most if not all of my polyps dying out. If there are any still in my tank, then they must be someplace I can't see and must not be releasing ephyra lately. Which is sad.
Oh well, maybe in a few years I will try to setup a tank just for Upside Down Jellyfish. Until then, I will just focus on my reef tank. And also make some improvements here and there to my freshwater community tank too.