Managing the Scope of User Stories in Scrum

I'd like to introduce an article, Writing the Product Backlog Just Enough and Just In Time by Mike Cohn on scrum Alliance's website.

His main point is that,

[e]ach product backlog item (usually reflected as a user story by teams I train or coach) should be captured just in time and in just-enough detail for the team to go from product backlog item to working, tested feature within a sprint.

And,

If the team will not work on this user story for six months there is no need to expand on what is already written. On the other hand, if the team hopes to start it in a few sprints, it is likely time for the UEDs to figure out the answers to the high-level questions listed above.