Recommended: Leave this un-checked.

Fixed-Size Video is the opposite of "Responsive" Video (you want your videos to be responsive.)

For any specific reason, if you want your video to be of an exact specific size, regardless of the size of the screen of the device it is being viewed on, then set this to "Yes" (check it).

But ideally, you want your video to be responsive - which means, your video should take on a size that fills up the section wherever you have placed your video. AND you want the video to resize based on the device on which it is being viewed. In that case, UN-CHECK this. And then the video will then resize on smaller devices, and grow bigger on larger desktop screens. And that's the ideal viewing experience for most web users.

For fixed-size videos, a couple of standard widths and heights are (width x height):

800 x 450
640 x 360