It's very important that you know how long you want your novel to be, whether its the size of Sherrilyn Kenyon's "Born of Fury", or maybe something smaller like Robert Louis Stevenson's "Treasure Island". Either way, knowing your desired word count is crucial.

Take J.K Rowling's Harry potter series for example: 

Harry Potter and the Philosopher's Stone - 76,944 words  
Harry Potter and the Chamber of Secrets - 85,141 words  
Harry Potter and the Prisoner of Azkaban - 107,253 words  
Harry Potter and the Goblet of Fire - 190,637 words  
Harry Potter and the Order of the Phoenix - 257,045 words  
Harry Potter and the Half-Blood Prince - 168,923  
Harry Potter and the Deathly Hallows - 198,227

The total word count for each of her books varies drastically, but each novel is filled to overflowing with adventure. The perfect balance has to be created; where there's never too many words and not enough adventure, nor too much adventure and not enough words.  In order to successfully outline your book it is important to aim at, or around, your genre's average word count. Here are some examples:

Fiction

Fantasy: 90,000 -125,00 words

Epic Fantasy: 180,000 - 200,000 words

Romance: 50,000 - 90,000 words

Mainstream Romance: 70,000 - 90,000 words

Science fiction: 90,000 - 125,000 words

Mysteries & Crime: 80,000 - 100,000 words

Thrillers: 80,000 - 100,000 words

Historical: 80,000 - 120,000 words

Horror: 80,000 - 100,000 words

Literary: 80,000 - 120,000 words

New age: 60,000 - 85,000 words

Western: 50,000 - 80,000 words

 

Non-Fiction 

Memoir & Biography: 80,000 - 100,000 words

Narrative Non-Fiction: 70,000 - 110,000 words

Self-help & How-to: 40,000 - 60,000 words

 

Age range

Adult: 80,000 - 125,000 words

Young Adult: 50,000 - 90,000 words

Middle Grade: 30,000 - 50,000 words

Children: 500-600 words

 

Source: www.savannahgilbo.com

 

Now, while aiming at the average word count is a good place to start, it's also important to realize that the amount of words provided is not limiting to your story. You don't have to land within the margins. 

The challenge: 

I want you to find a book that is around the size you picture your finished novel to be, then to look up it's word count. I'll bet that it's either way more, or way less then you anticipated. 

Do you see what I mean? You may have no idea how many words you're actually aiming for until you realize what size you want your novel to be, and thats where the fun starts.