Change width in the block

In D7 I got into trouble with the layout, the block was always too wide so I decided to change the CSS from this:

div.shoutbox-msg {
  padding: 4px;
}

To this (look at width, any other design might have different width):

div.shoutbox-msg {
  padding: 4px;
  width: 160px;
}

The best would be to give users an option in the admin interface to change this width or find any other solution for this problem.