Kicking Tires quotes "a source" that says that the exact size of the smaller tank is not yet set but that, "We're working on that." The new goal is to give the Volt a range of 360 miles without any help from the initial charge, so that would imply a tank of maybe eight or so gallons - still plenty for most drivers most days in a PHEV like this. While the aerodynamics of the Volt have proven to be much more important than weight when it comes to increasing the miles per gallon number, every bit counts. We're waiting on a reply from GM to see if the smaller fuel tank move is official.
UPDATE: GM's response was that they will share "the detailed technical information on the production vehicle at the appropriate time."
[Source: Kicking Tires]