When you say "VBox" you mean a VBoxContainer right?
How are you saving the speech dialog? If you're saving the dialog as strings then you shouldn't have to save the size of the box container. You could load the strings, construct the labels for them, and insert them into the box container, which should resize itself automatically. This should be reflected by the scroll container assuming the scroll container is of a fixed size.
That said, there are a few bugs with scroll containers not detecting changes in their children correctly (this seems to be an issue with containers in general). You may need to call the scroll container's
queue_sort method when its child box container is updated, like in response to the box container's