The CSS class name is .author-content

Control Panel > Styles > SELECT A FORUM STYLE
Once you are in the Edit or Add a Style page, modify Post Properties > .author-content

The default when not set is 130px.

You can modify that by adding the following CSS code to that group.
Code
width:130px;

Adjust the px width as you desire.

current developer of UBB.threads php forum software
current release: UBB.threads 8.0.1 // wip: UBB.threads 8.1.0
isaac @ id242.com // my forum @ CelicaHobby.com