This feature request was posted in the discussion forum, but as far as I can tell, it was never posted in the ideas forum.
Essentially, the request is to provide separate sliders for text background width and height in the essential graphics panel. Currently, there is only one slider (circled in red) to control both width and height, but the horizontal width is often too small and it would be very useful to be able to control it independently.
The only workaround I've found is to add an extra space at the beginning and end of each line of text, which for obvious reasons is not a great solution.
Original discussion here:
https://community.adobe.com/t5/premiere-pro-discussions/subtitles-appearance-background-size-how-to-...