The size of a layout (often referred to as its geometry ) is calculated automatically on the basis of the size of each of its children. The algorithm used takes account of hints provided by the children, and from the description of the layout itself. Hints are specified via the panes' initargs when they are created. The various pane classes have useful default values for these initargs.
CAPI User Guide (Macintosh version) - 30 Aug 2011