I’ve written a lot about Design Documents (dd) recently. This is a great template for a really large project – I’ve never used all of these sections but certainly a number of these categories are useful. It certainly gives you an idea for the scope of what a DD covers.

http://blogs.microsoft.co.il/gadib/2010/10/11/technical-design-document-template/

Just remember the design document is best if it’s as small as you can keep it.