Thank you again because I solved the other problem too, in fact I could scale and rotate the text perfectly!
I just draw the text with the default size and then I scale it depending on the rectangle size.

Just I changed the rectangle mapping from a QPainterPath to a QPolygonF, because I think it is faster (QPainterPath handles curves too, while QPolygonF handles just points). I converted it to a QPainterPath after the mapping.

Here is the code (may be it can be useful to someone else):
Qt Code: 
  1. {
  2.             // Rectangle mapping
  3.  
  4.             QLineF rectBase = QLineF(begin, end);
  5.             qreal angle = rectBase.angle(QLineF(QPointF(0, 0), QPointF(1, 0)));
  6.             QMatrix rotationMatrix;
  7.             rotationMatrix.translate(begin.x(), begin.y());
  8.             rotationMatrix.rotate(angle);
  9.             path.addPolygon(rotationMatrix.map(QPolygonF(textRectangle)));
  10.  
  11.             // Text mapping
  12.  
  13.             QFont font = QFont("courier");
  14.             // Take text size
  15.             QFontMetricsF fm(font);
  16.             QSizeF textSize = fm.size(0, text);
  17.             // Calculate how much to scale the text to fit the rectangle
  18.             qreal scaleX = textRectangle.width() / textSize.width();
  19.             qreal scaleY = textRectangle.height() / textSize.height();
  20.             // Apply the scale factors
  21.             rotationMatrix.scale(scaleX, -scaleY);
  22.             // Draw the text
  23.             QPainterPath textPath;
  24.             textPath.addText(QPointF(0, -fm.descent()), font, text);
  25.             path.addPath(rotationMatrix.map(textPath));
  26. }
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