What is the optimal size for a background video on a careers website homepage?

On the splash page, I'm looking at the below field.

This is to load a video to the background of your homepage, so it plays behind the search bar etc.

Question 1 - What is the optimal dimensions for the video so that it'll look good on mobile and desktop? The one we have uploaded looks very very zoomed in like only a thin central portion shows.

Question 2 - What is the optimal file size? The only we have uploaded will not play on a mobile or is very slow to load.