I've already written that, like other elements of the window, you can adjust the position and size of the rectangle in which the text is placed. In the DialogueScreen.(WindowScreenShared).xdb file, there is a link to HeroText.(WindowTextView).xdb, where you can adjust the size and position of the rectangle, and there are also several settings (ResizeOnTextSet and ResizeWidthOnTextSet) in it; it links to HeroText.(ForegroundTextString).xdb. In HeroText.(ForegroundTextString).xdb, there is a link to Text.(ForegroundTextStringShared).xdb, which in turn has a link to a TXT file where the text format is configured: 1) text size (including style); 2) position (left alignment, right alignment, center alignment); 3) text color.
I've seen this in the "Dark Messiah" scenario, in the dialogue with Sandro, before deciding whether to free Kha-Belekh or not. There, even without a mod, the interface of the cutscene is adapted to a widescreen (in real time), and the text is positioned up to the button. If you shrink the window, the text also shrinks along with the entire interface.
Added after 39 minutes
If you think about it, creating a text box for a 21:9 aspect ratio (from edge to button) is quite a challenge. You could strain your neck reading it.