Why isn't Sin City treated like more of a classic?

I liked the movie more than the comic, but that’s pretty faint praise. IIRC, Sin City was Miller’s first big project not to be edited by Denny O’Neil. I never would have guessed that O’Neil was the only thing making Miller’s writing as palatable as it was for so long, but that is my reluctant conclusion.

Comic-book Rorscach is never depicted as tall - he’s occasionally described as little. His voice is described as a monotone.

The tallest character in the story is Doctor Manhattan, even disregarding super powered size changes.

I think you’re missing Gerald’s point. There’s YA novels, and there’s non-YA novels, but they’re both still novels. Likewise, there are graphic novels for kids, and there are graphic novels for adults, but they’re still both graphic novels. The idea that “graphic novels are comics for grown ups” is basically a vestigial marketing gimmick left over from the '90s. These days, it doesn’t mean much besides, “comic book held together with something slightly more durable than staples.”