vBShout will automatically fill 100% of the width of the container it's placed within.
If that's the view you see, it must mean that some outside HTML tag is constraining its width. I don't know if anything can be done, though you could try editing dbtech_vbshout_shoutbox and changing <div class="collapse" style="padding-bottom:10px;"> to <div class="collapse" style="padding-bottom:10px; width:100%">
If that does not work, then there's nothing further I can do I'm afraid