QPlainTextEdit - document()->size() does not return the document size

#include <QtGui>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QPlainTextEdit TextWin;
    TextWin.setPlainText("this is a test \n Hello Hello Hello Hello\n World");
    TextWin.show();
    qDebug() << TextWin.document()->size()
}