edit the border of single posts

Hi there,

Can anyone help, I wanted to changen de width of my single posts. so I did but the problem now is that the text that is in the post, with the social media buttons, goes of the post and disappear.
I found that to change this I have to change the border of it, but when i try to locate it by inspecting the elements. it says that I should be in post-number varies.

the single.php says: <div id=”post-<?php the_ID(); ?>” <?php post_class(); ?>>

Does anyone know where I can find it?

thanks