After speaking with Qt support, the only way I found was to assign an empty icon to all tabs, and set the icon-size to the desired height.
By laurapons
... for (int i = 0; i < ui->toolBox->count(); i++){ ui->toolBox->setItemIcon( i, QIcon(":/shared/empty")); } ui->toolBox->setStyleSheet("QToolBox{ icon-size: 48px; }"); ...