Grabbing space for a widget

[Jonathan Hayward <christos jonathan hayward gmail com>]

I currently have a window that's laid out like this:

111111111111111
2222222222222222222222222222222
2222222222222222222222222222222

3333333 44444444444444444444444
3333333 44444444444444444444444
3333333 44444444444444444444444
3333333 44444444444444444444444
3333333
3333333 5555555555555555
3333333 5555555555555555

Widgets 1, 2, 3, and 5 should be about the same size regardless of the overall size of the window (apart from the width of 2).

I'd like to adapt my code so that 4 shrinks if the user shrinks the
window, and expands in size if the user expands the window. I can do
this if I know when the size has changed, and how many pixels 4 can
occupy given a window's new size.

Is there a way to ask "How much space can this widget occupy?
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