In Lightroom CC 6.4 on a Macbook pro w retina:
Video clips do not scale up to the full picture cell in the slideshow. They are shrunk.
If I adjust the resolution on the display (in Mac settings) to lowest possible, then the video clip fills the cell. Must be a bug in Lightroom where the video onlys shows max one video pixel on one display pixel. It doesn't scale up to the correct size.
As an alternative solution: exported pictures and video clips from Lightroom and imported them to "Pictures for OSX" and there everything worked as expected (pictures and video clips scale to the same size, while display is set to full resolution). But I wasn't able to hear the audio from the clips in the Pictures slideshow this way...
As far as I remember the following applies to picture slides, it probably will for video as well:
It does not zoom over 100%. Meaning if you want them to be full screen, there resolution needs to be at least the same, or of course higher as the resolution of you screen.
The functionality regarding videoclips in slideshows in Lightroom is too simple. In a slideshow i should be possible to zoom a video more than 100% as in the much simpler Photos app. The resulting slideshow in Photos looks very good.