Word length is key to being able to sell your manuscript. Agents and editors both take it into careful consideration. Lengths differ by genre. Here are some loose guidelines in the genres I both write and look for as an editor with City Owl Press:
Fantasy: 80,000 to 110,000 A little under is alright, but you will likely be asked to flesh it out some, over by much and you risk having two books instead of one. Not a bad problem to have, but understand that you may be asked to break it up into two books with full story ARC's worked into both.
Paranormal: 75,000 to 95,000. A little under is alright, but if you drop below 70,000, you may be asked to flesh it out, over 95,000 and it will likely get passed by.
Science-Fiction: 80,000 to 120,000. Rarely will you find sci-fi under 80k, it's just too involved for a shorter word count. On the longer side, you can go over, but don't go crazy. Know when your story and character ARC's need to complete, don't go beyond it. Too much more than 120 might mean you are burying the story (and the reader) in too much description and world building. While this genre requires a lot of both, it is a careful balance. If you find yourself going longer than 120k, think about writing it as a series instead of one book.
Historical Romance: 75,000 to 110,000. I would't really go under on this one because then it is likely that you aren't putting in enough 'historical' to give it the right feel, over and it becomes harder to sell. Even approaching the far end at 110k you need to make sure it is a very compelling novel with a good pace that doesn't lag.
Why didn't I put in romance except for as historical since it is one of the main things I read and look for as an editor? Because romance can (and should, for me personally) be worked into any genre and it shouldn't change the length much in my opinion.
For me, as both a reader and an editor, under 75k is too short to fully develop the story and over 100k is pushing it when it comes to keeping readers' attention. These numbers will vary from agent to agent or editor to editor, but not by much. There are a lot of other genres out there. For more info on other genre lengths, click here.