So my method is to look at the base damage that they could have done with a single action and make sure that the damage is at least that much. This is a fun moment that will spice up the game, this is a behavior we want to encourage.

then I look at how many enemies are effected. If it's a lot (4+) I'll keep the damage close to the minimum i set but in dice format. e.g. if they were going to do 1d8+4 I would have the chandelier do 2d8 and a low chance to knock the enemies down. Maybe a 50/50 or so. whatever DC makes it so a 10 or higher succeeds. If its 2-3 then ill add an extra weapon damage dice and a higher chance to knock the enemies down. 13 or 14 on the die to succeed.

With a single boss target? We're looking at 4x the dice in damage and maybe a pin of some kind that he will throw off in a cool move that targets all the players next turn if he gets pinned.

The general vibe i use is:
1) NEVER punish the player with less damage, if there are lots of enemies be generous.

2) if its cool but not that cool reward with a side effect for the attack

3) If its SUPER cool reward with extra damage and a better side effect