/**
 * Constructs the Widget object
 *
 * Basic constructor for the widget object.
 * Another line of pointless explanation, badly worded.
 *
 * @param  string   $id    The ID of the widget
 * @param  int      $size  The Size of the widget
 * @return void
 * @throws InvalidArgumentException
 */
public function __construct($id, $size) {
    $this->id = $id;
    $this->size = $size;
}

Argument $id is missing from the Docblock of __construct