Background Size is Responsible for Setting a Background picutre in a given
Contaiener in which you apply that special Background

/* This will make width as static and height as auto */
/* These three are quite same */ 
background-size: 100vw auto;
background-size: 100%;
background-size: cover;


/* for getting full width and height every time */ 
background-size: 100vw 100vh;