This problem is a rare bird, Trevor. The OP's screenshot shows normal sized thumbnails, but the pattern box panel has shrunk down and can't be resized.
Another example: Custom Pattern View Box in Fill Dialog Not Resizing Photoshop CC
It was a bug in the code Chris Cox mentioned he would fix, but a prefs reset usually puts it back to normal.
This is a better view of what you are seeing.
However I would like ciarrai to get back to us on this.