you can try this:
sized_btn = QPushButton( "Sized" )
sized_btn.setFixedHeight( 100 )
mBox = QMessageBox()
mBox.setText("BOB")
mBox.addButton(sized_btn, QMessageBox.AcceptRole)
mBox.setStandardButtons( QMessageBox.Discard | QMessageBox.Cancel)
mBox.setDefaultButton(sized_btn)
mBox.exec_()

