OS Details

OnePlus 6 (2018) has been launched in May 2018.
OnePlus 6 (2018) have Android 8.1 (Oreo) , upgradable to Android 9.0 (Pie); OxygenOS 9.0.2

Screen Dimensions

OnePlus 6 (2018) screen size is 6.28 inch with ~ 83.8% body ratio of actual device size. Physical dimensions for OnePlus 6 (2018) device is 6.13 x 2.97 x 0.31 INCH or ( 155.7 x 75.4 x 7.8 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.

OnePlus 6 (2018) resolution is 1080 x 2280 ^PX with ~ 402 pixel desnsity.
OnePlus 6 (2018) viewport size is 412 x 869 ^PX with ~ 153 actual pixel density, which means it have 2.625 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".

OnePlus 6 (2018) have 6.28" physical screen size and its resolution is about 1080 x 2280 Pixels with approximately 402 PPI pixel density. OnePlus 6 (2018) has viewport size 412 x 869 Pixels and its pixel ratio is about 2.625.

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

CSS Media Queries

OnePlus 6 (2018) Media Queries (In terms of Mobile only)

@media only screen and (min-width: 412px) and (max-width: 767px) { /* Your Styles... */ }

OnePlus 6 (2018) Min-Width Media Queries

@media only screen and (min-width: 412px) { /* Your Styles... */ }

OnePlus 6 (2018) Min-Height Media Queries

@media only screen and (min-height: 869px) { /* Your Styles... */ }

OnePlus 6 (2018) Landscape Media Queries

@media only screen and (min-width: 869px) and (orientation: landscape) { /* Your Styles... */ }

OnePlus 6 (2018) Portrait Media Queries

@media only screen and (min-width: 412px) and (orientation: portrait) { /* Your Styles... */ }

OnePlus 6 (2018) Device Width Media Queries

@media only screen and (min-device-width: 412px) and (orientation: portrait) { /* Your Styles... */ }
@media only screen and (min-device-width: 869px) and (orientation: landscape) { /* Your Styles... */ }

OnePlus 6 (2018) Retina Media Queries

@media
  only screen and (-webkit-min-device-pixel-ratio: 2.625),
  only screen and (   min--moz-device-pixel-ratio: 2.625),
  only screen and (     -o-min-device-pixel-ratio: 2.625/1),
  only screen and (        min-device-pixel-ratio: 2.625),
  only screen and (                min-resolution: 402dpi),
  only screen and (                min-resolution: 2.625dppx) { 
    /* 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 (for all 2x resolution supported devices)

@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 (for all 3x resolution supported devices)

@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 */
}
                                

OnePlus 6 (2018) Specifications

• Launch Date May 2018
• Android 8.1
• Physical Screen Size 6.28 INCH
• Resolution 1080 x 2280 PX
• Viewport Size 412 x 869 PX
• Pixel Density 402 PPI
• Device-Independent Pixels 153 PPI
• Pixel Ratio 2.625 xxhdpi
• Width 1080 PX
• Height 2280 PX
• Device Width 412 PX
• Device Height 869 PX
• Dimensions 155.7 x 75.4 x 7.8 mm (6.13 x 2.97 x 0.31 in)
• Weight 177 g - 6.24 oz