You can find the Boxed layout at the Slider settings → Size tab.
A slider with the Boxed layout fills the container it was put in. For instance, if you put the slider to a post, it will only fill the post container.
Allows the slider to scale down, so to be smaller than the set slider size.
Allows the slider to scale up, so to be wider than the set slider size.
Prevents the slider from getting smaller than the set px value.