macbook pro to external monitor 1920px via hdmi?
Yes, you will get 1920x1080 resolution. 

The native resolution of a 13" Retina Macbook Pro is actually much higher. From Apples website: 


  Retina display: 13.3-inch (diagonal) LED-backlit display with IPS technology; 2560-by-1600 resolution at 227 pixels per inch with support for millions of colors
  
  Native resolution: 2560 by 1600 pixels (Retina); scaled resolutions: 1680 by 1050, 1440 by 900, and 1024 by 640 pixels


The 13" retina MBP can also handle 1080P and higher resolutions through HDMI:


  HDMI video output
  
  Support for 1080p resolution at up to 60Hz
  
  Support for 3840-by-2160 resolution at 30Hz
  
  Support for 4096-by-2160 resolution at 24Hz


http://www.apple.com/macbook-pro/specs-retina/