Double spaced is industry standard if you're talking about a manuscript that you'll be submitting to publishers.

For more guidelines see:

[Shunn.net][1] 

[Writersdigest.com][2]




  [1]: http://www.shunn.net/format/story.html
  [2]: http://www.writersdigest.com/online-editor/what-are-the-guidelines-for-formating-a-manuscript