In edit mode, STM will automatically update its internal bounding box when the rectTransform is changed for ease of setting up a UI, but in a build, the bounds need to be manually updated, since the rect sizes will normally only change when enabling a UI, or changing the game's window's size.
Also, try experimenting with adding a "content size fitter" component to the STM object to have it control the horizontal/vertical layout group! It's been a while, but that may produce the results you need. (I could be interpreting the issue incorrectly)
For the window size change, you can call SuperTextMesh.RebuildAll() when changing the window size, and for enabling the UI, make sure the UI gameobject is enabled before your STM object! I tried to write in some automatic fixes for this, but Unity UI can be really fickle about it... You might have to call "Canvas.ForceUpdateCanvases()" before enabling STM if it still remains an issue.
I hope this helps! If these don't work, I'll try to fix it for the next update!