The camera view is affected by the projection matrix. The projection matrix allows you to set up the camera position, rotation and the size of visible space.
Games mostly use two modes of camera projection. The first one is the orthogonal mode, which can be used to draw 2D scenes on the screen. This mode doesn't use the perspective camera, so the object's distance doesn't affect its size. On the other hand, the perspective mode causes farther objects to be visibly smaller than the nearer objects.
This recipe will assume that the current screen dimensions are stored in the variables screen_width and screen_height. These variables might change their values when you resize the application window or change the screen resolution. You might want to update these variables on resize event.