Any place to set/adjust video frame buffer size

[Jeffff]

I'm building a QT Application on Freescale iMX6 platform.
For now, I could get the image from Camera, and send it present. However, the issue is: only small size of video is OK.
As far as I tested, if a video is 480 * 270, it is OK to present(display). While if a video is 640 * 360(or larger), it cannot.
Another interesting trial is : if I use a video 640 * 200, or 200 * 480, they are all OK to display. That means the bottleneck must be the frame buffer size. So, my question is: is there any place that I should set/modify to frame buffer size in QT?(Either on building stage or running stage).
Here is my current code for display:

void VideoProducer::updateFrame(QImage image)
{
    if(image.size() != mFormat.frameSize())
    {
        closeSurface();
        mFormat = QVideoSurfaceFormat( image.size(), QVideoFrame::PixelFormat::Format_RGB32);
        qDebug() << "Cam Image Size:" << mFormat.frameSize();
        if(!mSurface->start(mFormat)) {
            qDebug() << "VSurf Started Fail";
        }
    }
    if(!mSurface->present( QVideoFrame( image ) )){
        qDebug() << "Present Fail";
    }
}