"[I]Render in Std AVC/AAC Internet 1080p, from a source which is avi, screen capture in 12080x720.[/I]"
Not a solution to your OpenCL question but rendering 720 to 1080 is a complete waste of CPU, GPU and your time for no benefit to society. You should not uprez but rather render to match your source video in this case.
That's I'm trying to do generally. But in some cases, I have to upgrade the resolution size (capturing DVD to project them with bad quality Video Projector).
In this particulary case, it's only a test.
After that, I've observed that filesize depends of frame size. Because we are in France, the Internet is slow, and I have to optimize a lot the download filesize as much as possible.
You are not going to see any difference in rendering CPU vs GPU if all you are doing is dropping a clip onto the timeline and rendering because your GPU is not supported for AVC rendering.
You will only see a difference when you start adding FX, color correcting, etc. Then the GPU will be faster because it will improve your timeline playback that feeds the render step.