To render a specific area, while in Camera View,press Ctrl+B and drag. You can adjust the selection size by clicking-and-dragging the corners.
Only the specified area actually gets rendered, but the resulting image will be full-size.
To remove the selection, I deselect the Render Region checkbox in the Output Properties panel. Supposedly, Ctrl+Alt+B will also remove it, but I use that particular combination as a Windows speed key for starting Blender 3D, so it doesn't work for me! [Your results may vary].