#include <QtWidgets>
#include <QDebug>

int main (int a, char **b)
{
    QApplication app(a, b);
    QWidget w;
    w.show();
    qDebug() << "Before everything: min_size=" << w.minimumSize();

    w.setStyleSheet("QWidget { min-height: 50; max-height: 50;} ");
    qDebug() << "After setting constraints: min_size=" << w.minimumSize();

    w.setStyleSheet("QWidget { min-height: 50; max-height: 50;} ");
    qDebug() << "After removing stylesheet: min_size=" << w.minimumSize();

    return 0;
}