imageCapture = new QCameraImageCapture(camera);

    QImageEncoderSettings imageSettings = imageCapture->encodingSettings();
    qDebug() << imageCapture->supportedResolutions();
    qDebug() << imageSettings.resolution();

    // !!
    imageSettings.setResolution(QSize(640, 480));

    imageSettings.setQuality(QtMultimediaKit::VeryHighQuality);
    imageCapture->setEncodingSettings(imageSettings);

    camera->setViewfinder(ui->viewfinder);

    connect(imageCapture, SIGNAL(imageSaved(int, const QString &)), this, SLOT(slotCaptured(int, const QString &)));

    camera->setCaptureMode(QCamera::CaptureStillImage);
    camera->start();
...

void Camera::processCapturedImage(int requestId, const QString &file)
{
    Q_UNUSED(requestId);

    QImage image(file);

    qDebug("Captured %dx%d", image.width(), image.height());
}

(QSize(160, 120) ,  QSize(176, 144) ,  QSize(320, 240) ,  QSize(416, 240) ,  QSize(352, 288) ,  QSize(424, 240) ,  QSize(640, 360) ,  QSize(640, 480) ,  QSize(800, 448) ,  QSize(800, 600) ,  QSize(960, 544) ,  QSize(1280, 720) ,  QSize(1280, 800) )  
QSize(-1, -1) 
Captured 1280x800