How do a resize a layer to a percentage of the background?

Hi Ryan.  Do you mean like a montage?

Turn the rulers on, and drag out some guides that will snap to the centre of the image.

Select the Frame tool and drag out windows that snap to the guides like below.

What you do then is select one of the frame layers, and go File > Place Embedded, and chose the image you want to put in that quadrent.  It will automatically size itself to fit the frame.  It's a real time saver for that sort of project.

If you use View > Guides > New Guide Layout there are a number of presets that might work for you, but otherwise it's super simple to create the number of positions you need.  You can also use the first group of four as a template making it even easier.  Think about the end product.  I like to use the File > New > Print presets even when not printing.