OS Details

Samsung Galaxy Note9 has been launched in August 2018.
Samsung Galaxy Note9 have Android 8.1 (Oreo)

Screen Dimensions

Samsung Galaxy Note9 screen size is 6.4 inch with ~ 83.4% body ratio of actual device size. Physical dimensions for Samsung Galaxy Note9 device is 6.37 x 3.01 x 0.35 INCH or ( 161.9 x 76.4 x 8.8 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.

Samsung Galaxy Note9 resolution is 1440 x 2960 ^PX with ~ 516 pixel desnsity.
Samsung Galaxy Note9 viewport size is 414 x 846 ^PX with ~ 147 actual pixel density, which means it have 3.5 xxxhdpi 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".

Samsung Galaxy Note9 have 6.4" physical screen size and its resolution is about 1440 x 2960 Pixels with approximately 516 PPI pixel density. Samsung Galaxy Note9 has viewport size 414 x 846 Pixels and its pixel ratio is about 3.5.

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

CSS Media Queries

Samsung Galaxy Note9 Media Queries (In terms of Mobile only)

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

Samsung Galaxy Note9 Min-Width Media Queries

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

Samsung Galaxy Note9 Min-Height Media Queries

@media only screen and (min-height: 846px) { /* Your Styles... */ }

Samsung Galaxy Note9 Landscape Media Queries

@media only screen and (min-width: 846px) and (orientation: landscape) { /* Your Styles... */ }

Samsung Galaxy Note9 Portrait Media Queries

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

Samsung Galaxy Note9 Device Width Media Queries

@media only screen and (min-device-width: 414px) and (orientation: portrait) { /* Your Styles... */ }
@media only screen and (min-device-width: 846px) and (orientation: landscape) { /* Your Styles... */ }

Samsung Galaxy Note9 Retina Media Queries

@media
  only screen and (-webkit-min-device-pixel-ratio: 3.5),
  only screen and (   min--moz-device-pixel-ratio: 3.5),
  only screen and (     -o-min-device-pixel-ratio: 3.5/1),
  only screen and (        min-device-pixel-ratio: 3.5),
  only screen and (                min-resolution: 516dpi),
  only screen and (                min-resolution: 3.5dppx) { 
    /* 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 */
}
                                

Samsung Galaxy Note9 Specifications

• Launch Date August 2018
• Android 8.1
• Physical Screen Size 6.4 INCH
• Resolution 1440 x 2960 PX
• Viewport Size 414 x 846 PX
• Pixel Density 516 PPI
• Device-Independent Pixels 147 PPI
• Pixel Ratio 3.5 xxxhdpi
• Width 1440 PX
• Height 2960 PX
• Device Width 414 PX
• Device Height 846 PX
• Dimensions 161.9 x 76.4 x 8.8 mm (6.37 x 3.01 x 0.35 in)
• Weight 201 g - 7.09 oz