Sponsorship letter for my spouse's visa -- what to write?

Seems like this is a pretty important document -- I plan on giving a quick history of our relationship and our reasons for moving to the UK, along with our plans once we get there. Is this basically what is required? And as for length, is one page about right?

Yep - its pretty simple.  You've got it.  One page is about right.  Let them know where you plan to live right away and your plans for a job search since neither of you will have a job when you move.