The minimum length of a novel is 40,000 words. Anything shorter is not classified as a true novel. Most are actually longer, however, such as 50,000-words. To be a novel, a manuscript has to be of fiction genre or can be based on true happenings.
It can be a romance, science fiction, drama, or many other classifications. Many novels these days are as long as 100,000 words. When readers get involved in a novel, most like to keep reading so the longer ones are becoming more popular. There is also an advantage writers have to compiling their novels now and that is the use of the computer to research and to type it out into a manuscript with ease. Self publishing is another convenience.