Hi,
I am not sure if you have found the solution or not. Anyway here is my thought
When you insert a video in a Presentation, it creates a <p:pic> element in slidex.xml where x is the slide number in which you have inserted the slide.
Now this element contains a child element <a:xfrm> something like this
<a:xfrm>
<a:off x="3048000" y="2286000" />
<a:ext cx="62860000" cy="68100000" />
</a:xfrm>
You can change a:off and a:ext elements attributes to change the size.
Thanks
Ankush