edit the associated .xml file, find the section for the tag and add or modify the width attribute. For example, if I wanted to change the width of the categories combo box in the header section, I would open/edit header.xml and find the object tag for the Categories token

<Object>

    <Token>[Categories]</Token>

    <Settings>

        <Setting>

            <Name>CssClass</Name>

            <Value>SubHead</Value>

        </Setting>

    </Settings>

</Object>

I would then add a setting for the width

<Object>

    <Token>[Categories]</Token>

    <Settings>

        <Setting>

            <Name>CssClass</Name>

            <Value>SubHead</Value>

        </Setting>

        <Setting>

            <Name>Width</Name>

            <Value>300</Value>

        </Setting>

    </Settings>

</Object>

That would change the dropdown to be 300 pixels wide. 

You can refer to the Admin/User's Guide for a complete list of the tokens and what settings are available for each one. You can download the Guide on the Project's Downloads page.