I won't do the job for you but I'm willing to help.
You will need 2 blocks of code:

an image displayer
a mouse-event listener

To start, you may forget about the image displayer. You may concentrate on the mouse listener while you draw your rectangle anywhere on the screen.
Select a mouse listener library. There are many on pypi.org.
I propose pynput because it is easy to work with and is well documented.

read documentation (focus on "on_click")
write your code to implement your mouse listener. It's simple (less than 10 lines). At the end of your program, add a statement:
input(">")
run your program. Click anywhere on the screen and drag to another point. Release.
your on_click() function will be called twice (once for button press and once for button release). Record the two sets of X-Y coordinates (unit is pixels).
once the button is released, compute the size of the rectangle (in pixels).
press any key on the keyboard to end the program.

Once your program is working you may work on the imager. If the image is large, you may have to use a scaling factor to reduce it. You will have to introduce the scaling factor in your sizing equations.
When a program skeleton will exist, do not hesitate to ask questions.
Asking for help when there is no visible sweat will not bring you  many answers.
I'm trying to develop a code that open an image where you can select a point quit the mouse and drag to form a rectangle until you don't release the left button.
Then from python I should receive the starting coordinates and the height and width in pixel of the rectangle, how can I do it?
I saw that the packages argparse and cv2 can be used, but I don't really know how to approach it.
Draw a Rectangle over an image and get the coordinates in python