How can I increase the size of the viewport(I don't know the exact name of it) The frame in the middle. I tried several methods, but I failed. Only I can do is just zoom in and out, but that's not what I wanted. The SECOND picture is the way that I wanted to. Make the frame large without zoom-in or out. Does anyone know how to do it?
If I understood you question correctly, I believe changing the values in the focal length on the camera could solve your question.
Select the camera in the viewport, go to "Object Data Properties" (little green camera seen in image), and adjust the focal length. You can also adjust the size just below if you toggle down the "Camera" setting.
35mm is one of the standard sizes used in filming, same as 12mm but I always found that to distorted.