Hey! This should be doable, I know other users have used the text mesh's bounds to get the right size, so I'll look into why that isn't updating. The textMesh variable there is the actual mesh used by STM.
That said, I know that accessing a variable from a mesh directly can cause garbage, so I have some variables in STM for getting bounds data (which I use for drawing the blue outline, in-editor.
I finally updated the docs to the latest version here, as I changed some bounds-related stuff since. http://supertextmesh.com/docs/SuperTextMesh.html
Check out the "topLeftBounds", "topRightBounds" etc variables in there, see if those work for you?