Screenshot Dimension When I go to About This Mac --&gt; Displays, it shows I have a Built-in Retina Display with 2560 × 1600 display. However, when I try to take a screenshot with command + shift + 3, the dimensions for that picture turns into 2880 × 1800. However, if I use command + shift+ 4 and drag the mouse all the way to the bottom right, the dimensions show 1440 × 900, but it turns into 2880 × 1800 if I save it. Which one is the actual size of my screen/screenshot?
Your screen may be a physical size unrelated to the numbers above.
Your screenshot size is easily checked in Preview app - open the saved file and get info.

How can I stop my retina display from taking 2x sized screenshots?

Your on screen pixel approximation is controlled by the displays system preferences. What you choose there affects the retina size vs rendered size vs reported size.

Scaling all UI elements when using an external monitor
https://en.wikipedia.org/wiki/Resolution_independence
https://en.wikipedia.org/wiki/Retina_display
https://support.apple.com/en-us/HT202471

Based on how much reading and programming you do these links may be more or less technical than desired. Hopefully it helps set the stage for more leaning if needed.