The doc block for the MenuSelectWidget lists the width as a parameter, which stops it from being displayed in the autodocumentation however in code it is implemented as config.
Doc block:
* @param {number|string} [width] Width of the menu as a number of pixels or CSS string with unit * suffix, used by {@link OO.ui.mixin.ClippableElement ClippableElement}
In code:
// Properties this.width = config.width;
Pretty sure that @param in the doc block should just be @cfg