Canonically, his hair is first (and arguably best) described here:

“He stared at the vampire’s full black hair, […] the curls that barely touched the edge of the white collar.” – Interview with the Vampire

^But he might have trimmed it that night, he did dress up for the interview, too! So he did pay some attention to his appearance.

I can’t recall anywhere in canon that Louis wears a hair ribbon/tie, which would give us that it’s at least long enough for that, so I don’t think it’s necessarily even that long.

Re: Brad!Louis hair: When I read IWTV for the first time it was the summer before the movie came out; they were already teasing about the movie, we knew who was playing Louis, and I’d seen Brad in other things and then saw the movie itself, so I admit that Brad!Louis influenced my headcanon of him in the beginning; I would have answered Brad!Louis then. Especially bc  I have wavy-curly hair myself and had fought it most of my childhood, so Louis’ hair being described as silky was my idea of manageable (straight, slightly wavy) hair! 

^And this can’t be an accident that they included this very blatant non-consensual fondling of Louis’ hair right when he’s about to be punished by Santiago & Co. 

Since that time, I’ve read plenty of fanfic and seen plenty of fanart that depicts Louis with varying hair lengths and textures, and after private debates with @wicked-felina, I accepted canon!Louis hair as preferable, but really, I love it all. I have an ability to enjoy

contradictory

canon and fanfic, parallel processing them both.

Most recently I’ve been very supportive of Ezra Miller’s hair from Madame Bovary as Louis hair. It’s full, meaning VOLUME, it’s more wavy at the top and curlier further down, and just long enough to touch his collar:

If you want to see some more examples of more of a canon!Louis hair, see my #Louis has canon hair I know that will make at least 2 ppl I know very happy tag (and those 2 know who they are!)

As beauty is in the eye of the beholder, again, I think both are perfectly valid!