The FAQ (frequently asked questions) on Spoonflower's website gives clear instructions on how to resize your images to achieve the size you want. I'm not going to type them here because 1) it would most likely be deleted by a moderator for copyright reasons, and 2) it would be long. Here is a link to the page with the resizing info.
http://www.spoonflower.com/help#help2
Spoonflower also has a blog with lots of helpful info:
http://blog.spoonflower.com/2008/05/...-i-prepar.html
If you still have questions, you can contact Spoonflower directly through the website. It is a small company and they are very friendly and helpful.