How many words long should my book be?

Frequently new writers either write an overly short book or an overly long one. Since I have been asked this question, I decided to post the answer here.
First of all you really need to decide if you are going to submit it to a publisher. If this is your first book, you have to stay within guidelines. Publishers receive thousands of queries each day. If you do not meet their guidelines and they do not recognize your name as a published author, yours will be passed.
Second, if you are not submitting to a publisher, you need to research what the average word count is for your genre. Most novels run 50K - 90K words depending on genre. Here is the thing: if you are self-publishing and the book is huge, most people are not going to want to try it. If it is too small (less than 15,000 words) no one is going to want to pay for it.
In addition to editing, I also review newly self-published books for a book reviewing agency. We get to select from all the books that have been submitted to them, so I deliberately tend to avoid anything over 250 pages because the chances of me telling the author he/she is going to need a major rewrite instead of giving them the review they want are high. Which leads me to...
Third, most new authors repeat themselves and actually give a lot of detail about things that are unnecessary. For example, in the current crime novel I am currently editing, the author explains what project the primary company is working on. Understanding the project is important, but then the author proceeds to tell the reader about the project each time a new character gets involved with the company by explaining it to the new character in dialogue. These explanations need to take place "off screen" because the reader already knows. This book has a good plot and is interesting, but the repetition would make a reader want to skip those parts and may turn the reader off to the book in general.
New authors also tend to tell people what is going on instead of showing them. This means that it is going to be hard for your reader to slog through an overly long book.
Once you have finished writing, it is always a good idea to find an editor who will read through it, correct grammatical errors, and honestly tell you what you don't need and what you do. Yes, some books are long and capture the reader - Lord of the Rings, The Count of Monte Cristo for example - but unless you have a massive marketing plan in place, keep in mind the size of the book will be something potential readers will use to judge whether or not they want to purchase it.