Try to be friendlier to small screen sizes. Use 800x600 if width/height of 2/3rds is smaller than 800x600 (things don't look smaller than that size), use desktop size if it is for some reason smaller than 800x600

#include <QApplication>

#include "mainform.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MyWindow *mainWin = new MyWindow;
    mainWin->show();
    return app.exec();
}