OS Details

Google Pixel 3a has been launched in May 2019.
Google Pixel 3a have Android 9.0 (Pie)

Screen Dimensions

Google Pixel 3a screen size is 5.6 inch with ~ 75% body ratio of actual device size. Physical dimensions for Google Pixel 3a device is 5.96 x 2.76 x 0.32 INCH or ( 151.3 x 70.1 x 8.2 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.

Google Pixel 3a resolution is 1080 x 2220 ^PX with ~ 441 pixel desnsity.
Google Pixel 3a viewport size is 393 x 808 ^PX with ~ 160 actual pixel density, which means it have 2.75 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".

Google Pixel 3a have 5.6" physical screen size and its resolution is about 1080 x 2220 Pixels with approximately 441 PPI pixel density. Google Pixel 3a has viewport size 393 x 808 Pixels and its pixel ratio is about 2.75.

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

CSS Media Queries

Google Pixel 3a Media Queries (In terms of Mobile only)

@media only screen and (min-width: 393px) and (max-width: 767px) { /* Your Styles... */ }

Google Pixel 3a Min-Width Media Queries

@media only screen and (min-width: 393px) { /* Your Styles... */ }

Google Pixel 3a Min-Height Media Queries

@media only screen and (min-height: 808px) { /* Your Styles... */ }

Google Pixel 3a Landscape Media Queries

@media only screen and (min-width: 808px) and (orientation: landscape) { /* Your Styles... */ }

Google Pixel 3a Portrait Media Queries

@media only screen and (min-width: 393px) and (orientation: portrait) { /* Your Styles... */ }

Google Pixel 3a Device Width Media Queries

@media only screen and (min-device-width: 393px) and (orientation: portrait) { /* Your Styles... */ }
@media only screen and (min-device-width: 808px) and (orientation: landscape) { /* Your Styles... */ }

Google Pixel 3a Retina Media Queries

@media
  only screen and (-webkit-min-device-pixel-ratio: 2.75),
  only screen and (   min--moz-device-pixel-ratio: 2.75),
  only screen and (     -o-min-device-pixel-ratio: 2.75/1),
  only screen and (        min-device-pixel-ratio: 2.75),
  only screen and (                min-resolution: 441dpi),
  only screen and (                min-resolution: 2.75dppx) { 
    /* 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 */
}
                                

Google Pixel 3a Specifications

• Launch Date May 2019
• Android 9.0
• Physical Screen Size 5.6 INCH
• Resolution 1080 x 2220 PX
• Viewport Size 393 x 808 PX
• Pixel Density 441 PPI
• Device-Independent Pixels 160 PPI
• Pixel Ratio 2.75 xxhdpi
• Width 1080 PX
• Height 2220 PX
• Device Width 393 PX
• Device Height 808 PX
• Dimensions 151.3 x 70.1 x 8.2 mm (5.96 x 2.76 x 0.32 in)
• Weight 147 g - 5.19 oz