The size property of the VTTCue interface represents the size of the cue as a percentage of the video size.
A number representing the size of the cue as a percentage of the video size.
In the following example a new VTTCue is created, then the value of size is set to 50. The value is then printed to the console.
let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";
let cue = new VTTCue(0, 0.9, "Hildy!");
cue.size = 50;
console.log(cue.size);
track.addCue(cue);