There seem to be 3 settings that affect tab width.
tabstop, softtabstop, and shiftwidth
When indenting or dedenting using << or >>, shiftwidth determines how far to or from the left margin the line will move.
To change shiftwidth, use :set shiftwidth=n, where n is the number of spaces you want >> and << to move the line.
See also: css - How do I change tab size in Vim? - Stack Overflow