In “Sixteen” theme built-in slider in order to change the slider width, edit the file

"wp-content/themes/sixteen/css/main.css"

.slider-wrapper {
  width: 600px;
  border-top: solid 4px #615642;
  border-bottom: solid 4px #615642;
  margin-bottom: -40px;
  background: #111;
  box-shadow: 1px 1px 5px #4a4a4a;
}

@media (min-width: 1200px) { /*Stop the slider growing crazily*/

  #customizr-slider {
      max-width: 600px;
      margin-left:auto;
      margin-right:auto;
      }

}