You need to override boundingRect() to return enlarged rect. I did some sample code.
This worked for me for all rotations and pixel sizes. Here it is.
Guess this is what you wanted

Qt Code: 
  1. #include <QtGui>
  2.  
  3. class FloatText : public QGraphicsTextItem
  4. {
  5.       //Q_OBJECT;
  6.    public:
  7.       FloatText( QGraphicsScene *scene  )
  8.          : QGraphicsTextItem(0, scene)
  9.       {
  10.          setPlainText ( "human ");
  11.          QFont f =  font();
  12.          f.setPixelSize(40);
  13.          setFont(f);
  14.          rotate(-90);
  15.          setFlags(QGraphicsItem::ItemIsMovable);
  16.       }
  17.  
  18.       QRectF boundingRect() const
  19.       {
  20.          return QGraphicsTextItem::boundingRect().adjusted(-2,-2,+2,+2);
  21.       }
  22.  
  23.       void paint(QPainter *painter,const QStyleOptionGraphicsItem *option,QWidget *widget)
  24.       {
  25.          QGraphicsTextItem::paint(painter,option,widget);
  26.          // Isn't this strange to call update in paint event ?
  27.          //update();
  28.          painter->setPen(Qt::black);
  29.          painter->drawRect(boundingRect());
  30.       }
  31.  
  32. };
  33.  
  34. int main(int argc, char *argv[])
  35. {
  36.    QApplication app(argc,argv);
  37.    QGraphicsScene scene(0,0,1024,768);
  38.    FloatText t(&scene);
  39.    t.setPos(500,500);
  40.    QGraphicsView view(&scene);
  41.    view.show();
  42.    return app.exec();
  43. }
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