OS Details

Apple iPhone 6s Plus has been launched in September 2015.
Apple iPhone 6s Plus have iOS 9 , upgradable to iOS 10.0.2

Screen Dimensions

Apple iPhone 6s Plus screen size is 5.5 inch with ~ 67.7% body ratio of actual device size. Physical dimensions for Apple iPhone 6s Plus device is 6.23 x 3.07 x 0.29 INCH or ( 158.2 x 77.9 x 7.3 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 6s Plus resolution is 1080 x 1920 ^PX with ~ 401 pixel desnsity.
Apple iPhone 6s Plus viewport size is 414 x 736 ^PX with ~ 134 actual pixel density, which means it have 3 xxhdpi 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 6s Plus have 5.5" physical screen size and its resolution is about 1080 x 1920 Pixels with approximately 401 PPI pixel density. Apple iPhone 6s Plus has viewport size 414 x 736 Pixels and its pixel ratio is about 3.

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

CSS Media Queries

Apple iPhone 6s Plus Media Queries (In terms of Mobile only)

@media only screen and (min-width: 414px) and (max-width: 767px) { /* Your Styles... */ }

Apple iPhone 6s Plus Min-Width Media Queries

@media only screen and (min-width: 414px) { /* Your Styles... */ }

Apple iPhone 6s Plus Min-Height Media Queries

@media only screen and (min-height: 736px) { /* Your Styles... */ }

Apple iPhone 6s Plus Landscape Media Queries

@media only screen and (min-width: 736px) and (orientation: landscape) { /* Your Styles... */ }

Apple iPhone 6s Plus Portrait Media Queries

@media only screen and (min-width: 414px) and (orientation: portrait) { /* Your Styles... */ }

Apple iPhone 6s Plus Retina Media Queries

@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: 401dpi),
  only screen and (                min-resolution: 3dppx) { 
    /* 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 6s Plus Specifications

• Launch Date September 2015
• iOS 9
• Physical Screen Size 5.5 INCH
• Resolution 1080 x 1920 PX
• Viewport Size 414 x 736 PX
• Pixel Density 401 PPI
• Device-Independent Pixels 134 PPI
• Pixel Ratio 3 xxhdpi
• Width 1080 PX
• Height 1920 PX
• Device Width 414 PX
• Device Height 736 PX
• Dimensions 158.2 x 77.9 x 7.3 mm (6.23 x 3.07 x 0.29 in)
• Weight 192 g - 6.77 oz