I have changed the size of the the rectangular box of the camera - the box you can select on the 3D view - some time ago and I can't turn it back to the old size. Can you look at the pictures and help me, please? Thank you so much!
The render resolution in the render panel defines the upper limits in the dimensions of all renders through the camera, final or viewport.
You can also set a Render Border, using CtrlB in Object Mode, to restrict the rendered area in all views, (further restricted by the render resolution, when looking through the camera). In the 3D view, it's delineated with a red dashed frame.
In Object Mode, hitting CtrlB again allows you to resize the border, and AltCtrlB dismisses it.