qt set widget width according to layout'width

I have three instances of QWidget, WidgetMain and WidgetMerchandise and WidgetCentral. WidgetMain has BorderLayout, implemented in Border Layout Example. Now, WidgetMerchandise is on the west side of layout, WidgetCentral is set on Central side of layout.but I would like WidgetMerchandise to have width to 1/4 width of WidgetCentral. Do I have to reimplement sizeHint() and minimumSizeHit() or should I achieve this task using qt size policies?