How to auto adjust the div size for all mobile / tablet display formats?

This is called Responsive Web Development(RWD). To make page responsive to all device we need to use some basic fundamental such as:-

1. Set the viewport meta tag in head:

<meta name="viewport" content="width=device-width,height=device-height,initial-scale=1.0"/>

2.Use media queries.

Example:-

/* Smartphones (portrait and landscape) ----------- */
@media only screen
and (min-device-width : 320px)
and (max-device-width : 480px) {
/* Styles */
}

/* Smartphones (landscape) ----------- */
@media only screen
and (min-width : 321px) {
/* Styles */
}

/* Smartphones (portrait) ----------- */
@media only screen
and (max-width : 320px) {
/* Styles */
}

/* iPads (portrait and landscape) ----------- */
@media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px) {
/* Styles */
}

/* iPads (landscape) ----------- */
@media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px)
and (orientation : landscape) {
/* Styles */
}

/* iPads (portrait) ----------- */
@media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px)
and (orientation : portrait) {
/* Styles */
}

/* Desktops and laptops ----------- */
@media only screen
and (min-width : 1224px) {
/* Styles */
}

/* Large screens ----------- */
@media only screen
and (min-width : 1824px) {
/* Styles */
}

/* iPhone 4 ----------- */
@media
only screen and (-webkit-min-device-pixel-ratio : 1.5),
only screen and (min-device-pixel-ratio : 1.5) {
/* Styles */
}

3. Larger Device, Medium Devices & Small Devices media queries. (Work in my Scenarios.)

Below media queries for generic Device type: - Larger Device, Medium Devices & Small Devices. This is just basic media types which work for all of scenario & easy to handle code instead of using various media queries just need to care of three media type.

/*###Desktops, big landscape tablets and laptops(Large, Extra large)####*/
@media screen and (min-width: 1024px){
/*Style*/
}

/*###Tablet(medium)###*/
@media screen and (min-width : 768px) and (max-width : 1023px){
/*Style*/
}

/*### Smartphones (portrait and landscape)(small)### */
@media screen and (min-width : 0px) and (max-width : 767px){
/*Style*/
}