OS Details

Apple iPad Pro 12.9 (2018) has been launched in October 2018.
Apple iPad Pro 12.9 (2018) have iPadOS 13.1.2

Screen Dimensions

Apple iPad Pro 12.9 (2018) screen size is 12.9 inch with ~ 85.4% body ratio of actual device size. Physical dimensions for Apple iPad Pro 12.9 (2018) device is 11.05 x 8.46 x 0.23 INCH or ( 280.6 x 214.9 x 5.9 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 iPad Pro 12.9 (2018) resolution is 2048 x 2732 ^PX with ~ 264 pixel desnsity.
Apple iPad Pro 12.9 (2018) viewport size is 1024 x 1366 ^PX with ~ 132 actual pixel density, which means it have 2 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 iPad Pro 12.9 (2018) have 12.9" physical screen size and its resolution is about 2048 x 2732 Pixels with approximately 264 PPI pixel density. Apple iPad Pro 12.9 (2018) has viewport size 1024 x 1366 Pixels and its pixel ratio is about 2.

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

CSS Media Queries

Apple iPad Pro 12.9 (2018) Only Media Query which covers portrait to landscape width.

@media only screen and (min-width: 1024px) and (max-width: 1366px)  { /* Your Styles... */ }

Apple iPad Pro 12.9 (2018) Media Queries (In terms of Tablets only)

@media only screen and (min-width: 768px) and (max-width: 1024px)  { /* Your Styles... */ }

Apple iPad Pro 12.9 (2018) Min-Width Media Queries

@media only screen and (min-width: 1024px) { /* Your Styles... */ }

Apple iPad Pro 12.9 (2018) Min-Height Media Queries

@media only screen and (min-height: 1366px) { /* Your Styles... */ }

Apple iPad Pro 12.9 (2018) Landscape Media Queries

@media only screen and (min-width: 1366px) and (orientation: landscape) { /* Your Styles... */ }

Apple iPad Pro 12.9 (2018) Portrait Media Queries

@media only screen and (min-width: 1024px) and (orientation: portrait) { /* Your Styles... */ }

Apple iPad Pro 12.9 (2018) Retina Media Queries

@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: 264dpi),
  only screen and (                min-resolution: 2dppx) { 
    /* 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 iPad Pro 12.9 (2018) Specifications

• Launch Date October 2018
• iPadOS 13.1.2
• Physical Screen Size 12.9 INCH
• Resolution 2048 x 2732 PX
• Viewport Size 1024 x 1366 PX
• Pixel Density 264 PPI
• Device-Independent Pixels 132 PPI
• Pixel Ratio 2 xhdpi
• Width 2048 PX
• Height 2732 PX
• Device Width 1024 PX
• Device Height 1366 PX
• Dimensions 280.6 x 214.9 x 5.9 mm (11.05 x 8.46 x 0.23 in)
• Weight 631 g - 1.39 oz