There's CSS in the template that you are using overriding what the particle expects, so Use this code...
.mrt-bgvideo-wrapper video {
max-width: 99999px;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.