A quick search gives some guidelines for how long certain types of books generally are:
Novels: 40,000+ words.
Novella: 30,000 to 60,000 words. Short but complete book, longer than a short story but shorter than a novel.
Short stories: Shorter than 30,000, but often times, the shorter the better. I began “The Wishing Star” because of a short story contest, however, the contest only accepted works of 1000 words or less. Personally, I don’t have the skills yet to form a complete and award-worthy story in so few words, so “The Wishing Star” is for this blog only.
Flash Fiction: Some would say that contest wasn’t really for “short stories” but for flash fiction, which is around 53 to 1000 words.
Once your story is complete, you’ll know better where you fall and where you want to fall on this list. Novels, Novellas, Short Stories and Flash Fiction all have their place, and all sell in different places. You’ll want to figure out where you want to sell your work.
Also, take your genre and audience into account. Young adult, middle grade, and children’s books should generally be shorter. Science fiction and fantasy genres can run longer, because there’s usually more world building and depth to add.
Longer books can put people off. My best advice is to write your story in it’s entirety, then go back and see what can be cut.
Unless you already have a good idea of your genre and your goal length, put those worries aside for the moment and tell your story first. Finish the story and see where it best fits, then cut it down and edit it to match as best you can.
Remember, most readers are just looking for a good story and quality writing than worrying about the length.