macbook pro to external monitor 1920px via hdmi? I just got macbook pro 13inch retina and the default resolution is 1280px width, and maximum is 1600px.

If I connect this laptop to an external monitor via HDMI which has 1920px resolution, will the resolution really be 1920, since laptop's max is 1600?
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/