getting the allocated size of a widget

["Nick Watts" <fatalglory gmail com>]

I'm writing an image viewer application and I want to be able to scale the
image being shown to fit the window, whatever size that may be.  There's
other stuff in the window too so getting just the window size isn't an
option.

What's the easiest/correct way to get the current width and height of a
widget (in this case a scrolled window/viewport)?  Can it be obtained from
the widget's GtkAllocation?