This question came up a LOT at the conference I just attended. And every editor and agent said the same thing: "I never tell writers how long to make their story. Make the story as long as it needs to be. And not one single word longer." Several went on to qualify it: shorter is better, especially for first timers. So yeah, there are 90k novels out there. But most rookie manuscripts (I include myself here) could use some liposuction. No lie, I brought a chapter book. 3-page first chapter. They made me cut all but the last 5 paragraphs of those 3 pages. And of course they were right. It's like a roller coaster. Now it starts at the top of the hill, instead of on the long climb up.
So when an agent sees 90k in the query letter, they may wonder how much editing needs to be done. Unless the hook persuades them you need all those words to tell the story, or the sample chapter is so tight they believe you can keep the pacing going, they'll probably think twice about it.
And from the other side, a shorter book broadens your audience. A reluctant reader will run screaming from a 300-page book. Sure, there are kids who will dive right in, voracious readers looking for a thick tome to get lost in. So it depends on who you want to grab and hang onto. My sympathy kind of goes out to the kids who struggle, and my goal is to write for them, so that affects my choices.
