How do I change left margin setting for column set using column builder

I have an existing column set, but when I use it in an FSG report, the row descriptions are getting truncated.  Trying to increase the column width.  When in Column Builder, I see there is a left margin setting - this is what is used to print description for row sets, I believe.  I would like to increase the left margin setting, but the column builder doesn't seem to let me update that field.  Any idea how I can change this setting?  Thanks.