GtkButtonBox::set_child_size()

Hello at all,

the code snippet 

        $bbox = &new GtkHButtonBox();
        $bbox->set_child_size(15, 20);

returns an error : 

	Warning: GtkButtonBox::set_child_size() is deprecated: don't use this

The Manual suggests the following :

	GtkButtonBox::set_child_size

	void set_child_size(int min_width, int min_height);

	Warning! This method is DEPRECATED and should NOT be used in newly-written 	
	code. Use the style properties child-min-width/-height instead. 

How do I manipulate the style properties ogf the Box ?

Thank you !