Problem with window/widget size

[never_ever]

Hi,
I have mainWindow (that inherits by QMainWindow), and I have second class that inherits by QWidget and it is invoked by first class.
In second window I have QGraphicsView. In second class I need to use QGraphicsView size (ui.graphicsView()->size()), but this size depends on the place where I write

@
secondWindow = new SecondWindow();
@

mainwindow.cpp

@
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
{
ui.setupUi(this);
connect(ui.pbOpenFile, SIGNAL(clicked()),
this, SLOT(addFile()));

//first way/place
newWindow = new MyClass();
}

void MainWindow::addFile()
{
this->hide();
fileName = QFileDialog::getOpenFileName(this,
tr("Open file"), QDir::homePath(), tr("File TXT (*txt)"));

if(!fileName.isEmpty())
{
//second way
//newWindow = new MyClass();
/*
...
*/
}
else
this->show();
}
@

myclass.cpp

@
MyClass::MyClass(QWidget parent)
: QWidget(parent)
{
ui.setupUi(this);
showMaximized()
/
...
*/
}
@

When I write it in first place the second window shows before I choose file to open and I don't want that. I want that it will open after I choose the file. But when I write it in second place I get wrong size of QGraphicsView widget (size of not maximized window).
Any idea why it happens in that way? Maybe I