manudurgoni

var cameraZ = camera.position.z;
var planeZ = 5;
var distance = cameraZ - planeZ;
var aspect = viewWidth / viewHeight;
var vFov = camera.fov * Math.PI / 180;
var planeHeightAtDistance = 2 * Math.tan(vFov / 2) * distance;
var planeWidthAtDistance = planeHeightAtDistance * aspect;

// or