To get the width of the blog Header, sidebars, main column, sign into Dashboard. In the section for the relevant blog, click LAYOUT, then click EDIT TEMPLATE. This will open the template editor window. Search through the templates for the following:
            
       #header-wrapper {

    width:750px;
    margin:0 auto 10px;
    border:1px solid $bordercolor;
    }

       #main-wrapper {

    width: 400px;
    margin-left: 20px;
    float: left;
    word-wrap: break-word; /* fix for long text breaking sidebar float in IE */
    }

      #sidebar-wrapper {

    width: 150px;
    float: right;
    word-wrap: break-word; /* fix for long text breaking sidebar float in IE */
    }

These codes are from the template of this blog. The first block of codes tells me that the width of the Header is 750 pixels. The second block says the main column width is 400 pixels. The last two block of codes tells me that both the sidebars are of width 150 pixels.