It's not a public event, that's why it's not documented. I've opened a feature request to add these events https://github.com/bryntum/support/issues/4060 , but could you please tell us what exactly you want to achieve with that?
Be aware this event may be (and will be) renamed before become to be a public.
Is it the size change of that component which requires a refresh?
Because a size change could arise from other sources. Such as if the widget ever is configured with a flex instead of a width, and its owning element changes size. Maybe the browser window changes on a regular system, or orientation change on a mobile.
If the size is your concern, then directly listening to the resize event on the widget is the better option.