simple question,set width of QTreewidget or QDockWidget

[weixj2003ld]

I create a mainwindow by inheriting QMainWindow,and on the left of the mainwindow,I create a tree throw the following code,

QTreeWidget *projecttree=new QTreeWidget();
...
QDockWidget *dock=new QDockWidget (tr("project"),this);
dock->setWidget(projecttree);
addDockWidnget(Qt::LeftDockWidgetArea,dock);

QTreeWidget *projecttree=new QTreeWidget();
...
QDockWidget *dock=new QDockWidget (tr("project"),this);
dock->setWidget(projecttree);
addDockWidnget(Qt::LeftDockWidgetArea,dock);

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When I run my program.I find that the initial width of projectree is large, how to change it or how to set the initial width?