Hey!
Yeah, autowrap/vertical limit control the width/height. I'm considering changing those two separate float values to be a Vector2, but I'm not sure if that makes it any more useable?
If you add a super text mesh component to an object with a RectTransform, STM should automatically switch to UI mode, and use the RectTransform's bounds as the autowrap/vertical limit. Right now, the presence of a rectTransform is the only thing telling STM to act differently when on a canvas, so... that's actually a really good point. I'll see what I can do to make it so autoWrap/verticalLimit can be set with a rectTransform component while managing uiMode in a different way, but it'll be a bit delayed, as I have some bugs to fix first, sorry!
As for vertical best fit, try playing around with different "vertical limit modes" for now? I'll see what I can do to add a best fit mode that considers the vertical limit as well as the autowrap value!
-Kai