set_usize() sets a widget's size in pixels. Sizing methods should only be called at one level of the packing hierarchy; you may need to size either the container or its children, but doing both will create conflicts in the size requests made by child widgets internally, which in turn can lead to unexpected results.

If you need either parameter to be ignored, use -1.

See also: GtkAlignment, and the packing methods supplied by GtkBox and GtkTable.