It looks that a couple of things are happening here:
1. The option you are looking for is called "Trim", there's a checkbox right above the Render button where you can disable it. Disabling it will make that the render isn't trimmed (cropped) and you'll have an output the same size as you want.
2. The render result preview is that size, that doesn't mean the export will have that size, It only means that the rest of the render is out of bounds. You can move the result in the preview window with Right Mouse Button, and you can zoom in/out with the mouse wheel.