Photoshop :: How To Batch Add Watermark/logo To

I've shot 4 series of Pianno recitals, which amounts to about 160 final photos. These photos will be going to a website from where clients can order prints at various sizes. Most of the clients will be ordering 5x7, but there could be some that are bigger (8x10, 8.5x11,etc). All photos will need to be uploaded to this site at their maximum resolution to accomodate the largest prints - it does not support a feature to upload the same photo in multiple sizes for different print options. All photos will need to have my custom watermark/logo with my signature and the event name and date - this would be just a simple text with shadow in the bottom right corner of the photo.

In the past, I only needed to support 5x7 prints which made my job easy. I simply cropped all photos to 5x7 ratio and resized to 300DPI, I ended up with 2100x1500 pixels. I then created a layer with the logo and used the batch tool to automate the addition of the logo layer to each photo. The position and size of the logo layer always ended up in the exact same spot, since all photos were exactly the same size.

For my new assignment, I will need to leave the photos at their original maximum resolution to support large prints. I'm strugling now trying to figure out how to automate the addition of my watermark/logo layer so that it appears in the same position and size on all photos. Is there any way how I can automatically resize the logo layer to match each photo's size. That way if I have an image that was heavily cropped to isolate the subject into lets say 1400x1000pixels and another that is lets say 2800x2000 pixels, my logo would proportionally scale up or down to maintain its relative size and position to the background image.