When writing your rough draft it should be as long as it should be...if that makes any sense. Basically, your story will be as long as it's supposed to be. Once you have finished your first draft, then you can go back in and cut anything that is not necessary to moving the story along.
When I read the LinkedIn post from a writer working on her novel, and concerned with the size of her book, (a fantasy fiction by the way) I was surprised by the answers the other writers gave her. Some said they won't read a book longer than 300 pages, they didn't have time, or other's said they preferred short stories to long books. And still some said, have you ever seen the size of the Game of Thrones books? I knew this was a topic I could relate to.
Most fantasy fiction readers like really long books, in fact they like series' of really long books. The difference between writing with a romance niche and a fantasy one is "World Building".
I remember the last writing conference I went to, the speaker was from Random House. The conference was geared toward romance writers and she was looking for books with 70,000 or less words. She sat next to me during the breaks and she and I got to talking. I told her I write fantasy and my first book, Fable, was 120,000 words. She said that was normal. (Which I knew that already)
I researched what the size of your book should be and in my genre there is no boundary, but that doesn't mean you should leave in a bunch of info dump that's not moving the story along. Actually the average is about 120,000 words, so I was pretty much on track.
Usually when writing any type of world building, such as fantasy, and sci-fi, you need the extra to bring the reader into the new world. So basically if your world is not in "this one" then you can make the book longer.
If you are planning to traditionally publish that can also determine how long your book should be. A lot of times they will be looking for a certain sized book for their readers, like the woman from Random House. You can research the type of genre you write in and see what is the most accepted length.
This is nothing to stress about. First write your rough draft. Get the story out on paper, and then concern yourself with the word count. Sometimes you can split the book in two and create a trilogy or series. There are ways around it.
If there are writers on here who have questions about the subject, feel free to leave me a comment. We can always look up the correct answers together. But if you are like me and write fantasy, don't worry, you're probably giving the reader exactly what they want, a new world to explore.
Until next week, I hope everyone has a wonderful 4th celebration!