Dynamic font size in QLabel based on text length which is copied from http://www.qtforum.org/article/32092/dynamic-font-size-in-qlabel-based-on-text-length.html

dynamic_font_size_in_qlabel_based_on_text_length.cpp

int fit = false;
this->testLabel->setText("");

QLabel *myLabel = this->testLabel;
QFont myFont = myLabel->font();    

QString str = "this is the new string. this is the new string. this is the new string. super long. don't fit. please please please overflow the box";

while (!fit)
{
    QFontMetrics fm( myFont );
    QRect bound = fm.boundingRect(0,0, myLabel->width(), myLabel->height(), Qt::TextWordWrap | Qt::AlignLeft, str);
    
    if (bound.width() <= myLabel->width() &&
        bound.height() <= myLabel->height())
        fit = true;
    else
        myFont.setPointSize(myFont.pointSize() - 1);
}

myLabel->setFont(myFont);
myLabel->setText(str);