The Sienna is definitely going to be bigger than before, showing off both that wider track and a stretched wheelbase. Toyota even modified the body panels on this mule in order to fit the wheels and tires underneath it all. Patchy work is noticeable around the front door and longer rear overhang. The mirrors are then haphazardly stuck onto the doors, instead of the A-pillars where they live now.
All of Toyota's cars that moved to TNGA are more refined and sharper to drive than their predecessors, so we'll assume the new Toyota Sienna will end up feeling the same way. It certainly has plenty of room for improvement compared to the Pacifica and Odyssey currently winning the minivan wars. At this stage, though, the Sienna is only in mule form, so it'll most likely be a couple years before any kind of a reveal happens. A 2021 model year is speculated for the redesigned Sienna, but there's still no confirmation from Toyota on that. If that timeline is correct, though, it means the current Sienna will last 10 years between full redesigns. The typical Toyota lifecycle is six.