How can I stop my retina display from taking 2x sized screenshots?
Here is the most convenient solution
(Taken from http://www.quora.com/How-can-I-get-my-retina-Mac-to-not-take-screenshots-that-are-too-big and modified a bit)

Use OsX embedded Automator to downsize screenshots automatically after they taken.


Open /Applications/Automator
Create a "Folder Action"
At the top of the window, where it says "Folder Action receives file and folders added to", select "Desktop"
From the panel on the left, select "Files &amp; Folders" and drag the item "Filter Finder Items" to the right panel.
Add the following conditions to the Filter Finder Items action: Name begins with "Screen Shot"; Kind is image.
(Optional step: if you want to keep copy of originals) - From the Actions list on the left, find "Copy Finder Items" and drag it to the panel on the right. Set the destination folder for the image copy (for example: "To: Pictures")
In the Actions library on the left, click on "Photos" and drag the "Scale Images" action to the right panel.
In the Scale Images action, select "By Percentage" from the drop-down and set the value to 50.
Select File > Save and give the Folder Action a snappy name like "Smaller Screenshot Copy"




Here is saved workflow file