I installed AllVideo and it works fine playing videos.
I have a locally uploades mp4 at the moment.
But how do I shrink the size of it ?
On the phone it looks ok, but on the computer its too large, so it gets pixelated.

If you are using the "Responsive" template, the video will be stretched the full width of your container, which sound like it's your screen size. You'll need to put the video code in another container to prevent the "responsive" nature or change to a different template.