The first part of the answer comes down to your own experiences and tastes. To a certain extent the type of books you read will influence the type of books you want to write. If you like reading YA, Romance, Sci-Fi, Fantasy, Crime or Thrillers, then chances are this is what you will want to write. Most genres have particular guidelines for novel lengths, and it is important that you understand these before you finishing putting together the plot summary for your novel. If you don't, and write your novel to a different length than what is expected in that genre, the chances are as a first time writer, your novel will disappear into the ether.
So what is the typical word count for a genre? Children’s novels typically have between 25,000 and 40,000 words, as the stories are adapted to the capabilities of their audience. Adult novels naturally set the bar higher, with anything in most genres below 50,000 words considered short. YA tend to be in the 60,000-75,000 bracket, Crime 'whodunnits' around 100,000, whilst Fantasy novels are anywhere from 120,000+ words. These numbers may seem high, but they are necessary to ensure sufficient content to grab, and keep, the readers attention. Writing 100,000+ words can take years – but remember, no-one said being a writer was easy!
Secondly, understanding the expected length of your genre is critical at the outset, so you can ensure you develop enough content within the plot to meet these minimum expectations. This is where preparation, and development of sub plots, pays for itself. Remember as a new author to take me to write down how each story arc will develop at the outset; if it looks light, spend the time at the start to rethink aspects, as this will be easier to do then looking to extend when you have finished.
The key thing to remember as a first time author is not you will not be treated the same as established authors in your genre. Just because King or Rowling wrote a short story and it was published and successful, doesn't mean your short story in that genre will be viewed the same. Established authors already have a fan base and a publishing network who will allow them to experiment or stray away from the expectations of the genre. As a first time author you do not get that luxury. Stick to the formula, expand and develop your plot until you reach the ball park word count.
The next point relates to your story, and doing it justice. If your plot summary is well developed, and has numerous sub plots or interesting diversions focusing away from the main characters, then it could easily run above the minimum threshold. As a first time writer I know the temptation is there; in the eagerness to get your story published and become a 'published author', aspects are removed so the story is the bare minimum. This temptation must be resisted. The reason is simple – it is your story, and therefore you need to tell it in full. Of course this doesn't mean writing rubbish, for the sake of word count, and during the editing phase anything that is irrelevant or poor should be removed. But if the story in your mind is 250,000 words of excellent content and plot, don't strip this back to 100,000 words to save time and get it published quickly. You only have the chance to write your story once – write it in full. If it takes you another 100,000 words and another year's work to tell the story fully, then do it. Your readers will thank you for it, and more importantly, the satisfaction you will have at the end will make it all worthwhile!
Of course, as Salvation is circa 270,000 words and has taken 3 years of my life, I would say that!
So what is the answer to how long your story should be? Well, as long as you meet the minimum expectations of the genre you are writing in, then as long as the story needs to be. I strongly believe as a writer you owe it to yourself to tell the story as you see it your mind and write how you want to read. Finally, whether your story takes 3 months and is 50,000 words, or 3 years and is 300,000 remember the most important thing of all - enjoy it! You are a writer, and that quite frankly, is awesome!