Specify dimensions of video embed?

Example:

vs gif

Stretching it looks bad, but I like the filesize efficiency of webm (476KB vs 3MB). With images, you can use image tags to override the defaults and specify width/height but I haven’t found a way to do it with video (tried video and iframe tags). Also, is there any way to specify loop/autoplay settings? Essentially, I want the simple usability of gif but with the efficiency of webm.

Just fixing gfycat embeds would solve this problem.