Altering the frame size around the "tree" in Threaded View

[Mandi]

I recently upgraded to 4.0.3, and we have a return of the scrollable frame around the discussion "tree" at the top of a thread. I would like to enlarge it, so more lines are visible. (And this has also been a community request.)

Where do I look for this item? I'm a competent tweaker, I don't need explicit instructions - just a point to the right template or script. Thanks!!