Hi Mirek

If its for your movie you might need to use an actual image plane rarther than a nurbs plane because your trying to match the resolution of the footage youve taken to a CG character.

In the render globals set up the resolution to the resolotuin that you want, also set up a CG camera with the settings that you used for the video camera (this helps to get the same perspective distortion focal attributes for the digital camera so it matches the real one you used)

Also in the image plane attributes click the set to image resolotion button (it might not be called that as I cant quite remember but its a big button)

With the camera set up you can then render from this camera (omiting the image plane of course) it should save you lots of headaches in the character not matching the footage.