The render result is correct but when in the viewer node the render is going off screen I'm trying to put my render with a background image which worked the first time, but when I enlarged the resolution of the background image in photoshop to twice its size (from 4368 × 2084 to 8736 × 4168) and put it back in blender (and after doubling the size of the render resolution to match) the render is going off screen as shown in the pictures. How do I fix this?

Here's a link to screenshots: https://www.flickr.com/photos/163507419@N03/?

first one shows how I want it to look (this was when it worked at the 4368 × 2084 resolution), the others show what it looks like now






thanks in advance
Without having your .blend and image files at hand, I can't tell for sure what is happening. 

However, usually a "Scale" node (found under "Distort"), set to "Render Size" and applied just after the Image input, should guarantee that RenderLayers and external images have matching sizes. (Note: if the image is smaller than the render layers, it will be stretched, losing sharpness in the process) 



I don't know, however, why they don't match already if – as you say – you already changed the resolutions in both the image and Blender. I can just suggest to check again.