Interesting. I too would have said longer fic was better, because a lot of times shorter fic rely heavily on you already knowing the characters and don't have enough time to establish them in their own right. But I guess if you're really definitely in it just for the trope, that doesn't matter. (Honestly if I'm looking for tropefic, I usually look for novelette length, because yeah I don't want to commit to a ton of backstory, but I want enough story for it to be worth it, too. But in fanfic, novelette counts as "long" :P)