The background-size property in 
background-size: auto|length|cover|contain|initial|inherit;
auto: This is a default value. It is used to display the original image with its original size.
length:  We use this to define the length and width of the background image. The first value should be considered width and the second value as height.
cover: We use this to resize the background image so that it can easily fit in the container.
contain: It is used to resize the background image to ensure that it is visible.
percentage: It is used to set the background image in terms of the percentage of the parent element.
initial: This sets the value to its default value.
inherit: This inherits property from its parent value.
The following is the basic background-size property.
In this example, we have set the width of the border image as 200px. We can also set the width according to our own choice.