I would first consult your theme stylist/developer. That looks to be a template edit, and I have to ask more information to help. What style theme are you using and, which phpbb version is installed?
Also could you please supply a screenshot (or hyperlink me to guest/demo view) of what you are currently looking at, so that I may be sure of what the real objective is. I think I understand what you want, but doesn't many styles already have a sidebar showing poster information? Yea, let me see what we're working with here please. And I can supply the template edits.
If you already understand template edits, then a simple html
<hr> is a standard to display a line 100% width of screen.
Also, css may be used here if you're better with CSS, and css is more recommended. You could do a