The length of a novel should depend on two things, and two things only:
1) It should be long enough to qualify as a novel; And
2) It should be just long enough to tell your story.
Too many authors try to stretch their novels into 200,000-word epics, only to bore their readers to tears. Others try to get the entire story over with in 50,000 words, leaving out valuable information. A novel should be just long enough to tell your story, but long enough so that all of the details are included.
Even the shortest novels, however, should be at least 50,000 words. Any shorter than that, and the novel becomes a novella. Anything less than 10,000 words is a short story. Although there really are no set "rules" for length of a manuscript, 50,0000-150,000 words is a safe bet. If your novel is more than 150,000 words, you might consider considering splitting it into two parts, creating a sequel.
That said, there are other factors which can influence the length of your novel. Pacing, characters and action are just a few, combined with the complexity of the subject matter. For example, in Tom Clancy's novels, he has to explain the complicated military jargon as well as the construction of planes and tanks. Therefore, his novels are much longer than 150,000 words. The same could be said for Jurassic Park , which uses in-depth scientific explanations.
Some authors choose to outline their plots before they begin writing, and using this technique, they can usually tell how long their novel will be before they even sit down to write. I never use an outline – I prefer to wing it – so the length usually comes as a surprise to me once I've finished. I judge the pace of the novel as I write, and I go over it chapter-by-chapter to make sure that I've written each scene as concisely and briefly as possible while still delivering the full effect.
For beginning reporters, your best bet is to just continue writing until you get a feel for length. Write short stories to practice telling a story in fewer words and work on condensing sentences into their purest form. It's an art – that, I'll admit – but once you have a sense of your own abilities as a writer, it will be second nature.