I agree with others that it depends on face shape/cut/type of hair.
I've seen some larger women with long hair who can pull it off really well. I've also seen it look really bad (the example I'm thinking of the woman had a very round face and super straight long hair it might have just been the cut too but it didn't match very if that makes sense?). It seems like the styles that I found the most flattering that were longer tended to have more body to them (wavy or curly). However, I could also imagine a scenario where someone had "too much" hair so in the end you just have to experiment a bit.
I guess long story short, maybe talk to a hair dresser and get their opinion. For me I always preferred to have short hair when I was larger. I have a round face but never carried a ton of weight in my face either. Short hair gave me a bit more volume but not an overwhelming amount. Also, when I have long hair that almost always means a ponytail the vast majority of the time and me at my highest with a ponytail was not the most flattering site in the world mostly because it would make my body look a lot bigger than my head.

So maybe if I wasn't so lazy I couldn't pulled off long hair better.
ETA: DH always prefers me with short hair no matter the weight but I've had other boyfriends (obviously before meeting him!) that liked me with long hair better regardless of weight so who knows!!!
