A new paper from Elon Musk (written before the Falcon Heavy launch, but published this month) with lots of detail on the BFR.
“Our updated design leverages a smaller vehicle, still pretty big but a single vehicle that can do everything that’s needed for greater Earth orbit activity. Essentially we want to make our current vehicles redundant. We want to have one system—one booster and one ship—that replaces Falcon 9, Falcon Heavy and Dragon. If we can do that, then all the resources that are used for Falcon 9, Falcon Heavy and Dragon can be applied to this system.”