OS Details

Apple iPhone SE has been launched in March 2016.
Apple iPhone SE have iOS 9.3.2, up to iOS 13.4

Screen Dimensions

Apple iPhone SE screen size is 4.0 inch with ~ 60.8% body ratio of actual device size. Physical dimensions for Apple iPhone SE device is 4.87 x 2.31 x 0.30 INCH or ( 123.8 x 58.6 x 7.6 MM).

Displaying of Units are in this order "Height x Width x Thickness"

Screen Pixel Density and CSS Pixel Ratio

"Pixel" is smallest unit/element of any display.

The total number of pixels that fit into an inch is referred to as "Screen Density" or "Pixel Density" and it measured as "Pixels Per Inch".
Pixel Desnsity have limits to display Pixels Per Inch depends on different screen sizes. When number of pixels per inch increases than it's screen-size limit, the Display resolution will increase but actual device width/height in pixels remain same. Actual device pixels are known as Device Independent Pixels or CSS Pixel Ratio.

Apple iPhone SE resolution is 640 x 1136 ^PX with ~ 326 pixel desnsity.
Apple iPhone SE viewport size is 320 x 568 ^PX with ~ 163 actual pixel density, which means it have 2.0 xhdpi display pixel density.

Screen Resolution and Viewport

The sum of pixels which are displaying on a device is called as "Screen Resolution". and sum of actual pixels of any device is reffered to as "Viewport".

Apple iPhone SE have 4.0" physical screen size and its resolution is about 640 x 1136 Pixels with approximately 326 PPI pixel density. Apple iPhone SE has viewport size 320 x 568 Pixels and its pixel ratio is about 2.0.

For better understanding screen actual sizes, viewport sizes, display resolution and about their density click here.

CSS Media Queries

Apple iPhone SE Media Queries (In terms of Mobile only)

@media only screen and (min-width: 320px) and (max-width: 767px) { /* Your Styles... */ }

Apple iPhone SE Min-Width Media Queries

@media only screen and (min-width: 320px) { /* Your Styles... */ }

Apple iPhone SE Min-Height Media Queries

@media only screen and (min-height: 568px) { /* Your Styles... */ }

Apple iPhone SE Landscape Media Queries

@media only screen and (min-width: 568px) and (orientation: landscape) { /* Your Styles... */ }

Apple iPhone SE Portrait Media Queries

@media only screen and (min-width: 320px) and (orientation: portrait) { /* Your Styles... */ }

Apple iPhone SE Retina Media Queries

@media
  only screen and (-webkit-min-device-pixel-ratio: 2.0),
  only screen and (   min--moz-device-pixel-ratio: 2.0),
  only screen and (     -o-min-device-pixel-ratio: 2.0/1),
  only screen and (        min-device-pixel-ratio: 2.0),
  only screen and (                min-resolution: 326dpi),
  only screen and (                min-resolution: 2.0dppx) { 
    /* Retina styles here */
}
                                

Retina is actually based upon device pixel ratio. Devices mostly have 2x or 3x display so you can use general retina media queries to display high resolution content on all kind of devices. Retina 2x and Retina 3x media queries given below:

Retina 2x Media Query

@media
  only screen and (-webkit-min-device-pixel-ratio: 2),
  only screen and (   min--moz-device-pixel-ratio: 2),
  only screen and (     -o-min-device-pixel-ratio: 2/1),
  only screen and (        min-device-pixel-ratio: 2),
  only screen and (                min-resolution: 192dpi),
  only screen and (                min-resolution: 2dppx) { 
    /* Retina styles here */
}
                                

Retina 3x Media Query

@media
  only screen and (-webkit-min-device-pixel-ratio: 3),
  only screen and (   min--moz-device-pixel-ratio: 3),
  only screen and (     -o-min-device-pixel-ratio: 3/1),
  only screen and (        min-device-pixel-ratio: 3),
  only screen and (                min-resolution: 384dpi),
  only screen and (                min-resolution: 3dppx) { 
    /* Retina styles here */
}
                                

Apple iPhone SE Specifications

• Launch Date March 2016
• iOS 9.3.2
• Physical Screen Size 4.0 INCH
• Resolution 640 x 1136 PX
• Viewport Size 320 x 568 PX
• Pixel Density 326 PPI
• Device-Independent Pixels 163 PPI
• Pixel Ratio 2.0 xhdpi
• Width 640 PX
• Height 1136 PX
• Device Width 320 PX
• Device Height 568 PX
• Dimensions 123.8 x 58.6 x 7.6 mm (4.87 x 2.31 x 0.30 in)
• Weight 113 g - 3.99 oz