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 cue1 = new VTTCue(0, 0.9, "Hildy!");
cue1.size = 50;
console.log(cue1.size);

track.addCue(cue1);