user38298: Using command :
ffmpeg -i source_video_path.mp4 -vf "subtitles=srt_source.srt:force_style='OutlineColour=&H80000000,BorderStyle=4,BackColour=&H80000000,Outline=0,Shadow=0,MarginV=25,Fontname=Arial,Fontsize=10,Alignment=2'" video_destination.mp4

I get

Ideally I would like to increase the subtitles background padding - especially horizontally.

user26758: You need to increase value of shadow, like "Shadow=10" and you will add padding to your BG.

Here you can read more about this.
As I know, no way to change x,y axis separately for BG size.

38298: <p>Using command :</p>
<pre><code>ffmpeg -i source_video_path.mp4 -vf &quot;subtitles=srt_source.srt:force_style='OutlineColour=&amp;H80000000,BorderStyle=4,BackColour=&amp;H80000000,Outline=0,Shadow=0,MarginV=25,Fontname=Arial,Fontsize=10,Alignment=2'&quot; video_destination.mp4
</code></pre>
<p>I get</p>
<p><a href="https://i.stack.imgur.com/uqgcb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uqgcb.png" alt="enter image description here" /></a></p>
<p>Ideally I would like to increase the subtitles background padding - especially horizontally.</p>
 26758: <p>You need to increase value of shadow, like <code>&quot;Shadow=10&quot;</code> and you will add padding to your BG.</p>
<p><a href="https://i.stack.imgur.com/1ah9t.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1ah9t.png" alt="enter image description here" /></a></p>
<p><a href="https://stackoverflow.com/questions/25870169/how-to-set-background-to-subtitle-in-ffmpeg">Here</a> you can read more about this.</p>
<p>As I know, no way to change x,y axis separately for BG size.</p>
