This CSS code, put at the end of the default style sheet, will help control the width of the container DIV which displays the products and other content.

/* -------------------------------------------
   Location of main content
-------------------------------------------*/
#container {
    /* margin:20px 200px 50px 200px !important; */
    width: 960px;
    /* margin-left: auto ;
    margin-right: auto ; */
    margin:20px auto 50px auto !important;

}

(This info is based on the Figero theme)