• WordPress.com Theme: Twenty Eleven CSS Style Sheet Modification. Change Site Title,Description,Post Title,Comments,Menus,Sidebar and More. – All About Basic
• Twenty Eleven Theme · BuddyPress Codex

ไปที่ Appearance | Customize | Additional CSS แล้วเพิ่ม CSS ดังนี้

#page {
  max-width: 1300px;
}

/* One column */
.one-column #page {
  max-width: 1300px;
}