Hey Chao,
there is no back-end setting for changing the sidebar width, for the sake of preserving the theme’s visual identity; if, however, you wish to decrease the width of the sidebar, you can add this code to the “Style: Add Custom CSS” tab of the Theme Options panel:
#sidebar { width: 35% !important; }
replacing the “35” with the figure you need.
Please note that this works only for values below 35%, as for everything above 35% additional changes in the CSS would be needed. The latter, however, should not be very problematic for a freelancer who is familiar with WordPress and CSS.
Sincerely,
Andrew / Satori Studio