I haven't tried it yet, but with the new Pin in version 197 you should be able to scale pinned objects with the parent object. So you need to pin all your buttons to the popup menu, and then scale it with Tween (Two properties) -> Size.
Another option is to use Tween Value, say from 0 to 1. And while this tween is running, set the scale for the menu and all other objects to Menu.Tween.value("tag")