1) Crop the image in such that you only see the screen and not area around it.
2) Open the image by Paint software and change the pixel size to 250*250 pixels in x and y directions.
3) Go to this following link to access Google Colab notebook: https://colab.research.google.com/drive/1ACnPPmeC_omVekJ8MCrKFqpZSw7pFCbO#scrollTo=TEblMdGsluq-
4. Log-in to Google account with following credentials:
Username: frsic0927@gmail.com
Password: FRSIC12345
4. Upload your photo in Drive -> MyDrive -> "Name_of_the_photo".
4) Insert the name of image into the Jupyter notebook Python code in the first block as '/content/drive/Name_of_the_photo.jpg' (Section: #Load the image).
5) Run the code. It will perform a Gaussian fitting based on the number of pixels in x and y directions.
6) If you need results in mm and not in pixel you should calculate the number of pixels per mm and add the information in block 4 (Section: #Conversion factors: pixels per mm). The large screen is (250/100=2.5 pixel/mm) and the small screen is (250/75=3.33) 
7) Then Run block number 4