QMdiArea subwindow size after cascadeSubWindows()

Question

I try to make my first MDI-app. I've made mainwindow with QMdiArea as centralwidget. I need to add subwindow ProjectWindow with big QTableWidget. But size of added subwindow is 220x64 instead of 800x600. 220x64 is sizeHint of ProjectWindow and SubWindow but i have no idea, how it can be chanched. I can change size after adding subwindow, but after using cascadeSubWindows() all subwindows become smaller again. Code is below:

ProjectWindow *pw=new ProjectWindow;
ui->mdiArea->addSubWindow(pw);
pw->setAttribute(Qt::WA_DeleteOnClose);
pw->show();
ui->mdiArea->subWindowList().last()->resize(QSize(800,600));
//ui->mdiArea->cascadeSubWindows();