Ok to make this more easier to understand I've put up some pictures of what I'm trying to do:

Let's say this is the original video of a big resolution:



I wan't to CUT out a part of this video:



I have the new cut-out video, and I want to export it:




I'm not trying to make a Zoom-in effect, because I will be exporting this new video in its own size. So if I cut out 400x200 pixels, the new video will contain this cutout and be of these dimensions.