Pontifications

Read the whole thing Shape Up: Stop Running in Circles and Ship Work that Matters. Love the shaping concept: “Second, we shape the work before giving it to a team. A small senior group works in parallel to the cycle teams. They define the key elements of a solution before we consider a project ready to bet on. Projects are defined at the right level of abstraction: concrete enough that the teams know what to do, yet abstract enough that they have room to work out the interesting details themselves.”
I think 6-8 weeks like Mozilla and Basecamp does is about right.
And there is no one size fits all; each software team will have its own process for scoping and breaking down features into manageable as well as useable chunks.