[Lynne]

Go to your admin cp. Click Styles & Templates, then Search in Templates. In the Top Search for Text box, enter ".postbitlegacy .userinfo" (no quotes). Hit Find. It will show the templates it is in in the various styles. Find the style you are interested in and double-click on the template (in this case, the template is postbit.css). It will open and the word will already be input into the Search in Template field so just hit the Find button next to it. Then it will show you the class:

.postbitlegacy .userinfo {
    float:{vb:stylevar left};
    position: relative;
    width:{vb:math {vb:stylevar postbitlegacy_userinfo_width}-{vb:math {vb:stylevar padding}*2}}; 
    padding: 2px {vb:stylevar padding} {vb:math {vb:stylevar padding}/2};
    font-size: {vb:stylevar small_fontSize};
    display:block;
        background: {vb:stylevar postbit_userinfo_background};
    clear:both;
}

From that you can see the width is setting using some math based around the stylevar "postbitlegacy_userinfo_width".