MacOS – Why Retina screenshots are twice as large as chosen area

It is impossible to deliver an "answer" to this question. You are asking for something that is purely opinion-based. I have friends who use super small fonts and it's very comfortable for them. I also have friends who use larger fonts, and that works best for them. I personally use a variety of font sizes depending on what Mac I am on and what App I'm using. Your best best is to work with whatever font size suits you, despite what anybody else tells you.

There is no "one-size-fits-all" answer to your question.

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.

1. Open /Applications/Automator
2. Create a "Folder Action"
3. At the top of the window, where it says "Folder Action receives file and folders added to", select "Desktop"
4. From the panel on the left, select "Files & Folders" and drag the item "Filter Finder Items" to the right panel.
5. Add the following conditions to the Filter Finder Items action: Name begins with "Screen Shot"; Kind is image.
6. (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")
7. In the Actions library on the left, click on "Photos" and drag the "Scale Images" action to the right panel.
8. In the Scale Images action, select "By Percentage" from the drop-down and set the value to 50.
9. Select File > Save and give the Folder Action a snappy name like "Smaller Screenshot Copy"

Here is saved workflow file