navigator and 100%

It's the Document's relation to your screen. If you "Fit to screen" Ctrl or Cmd 0, the red box surrounds the Image saying "you are seeing it all in your document window". For a large document that would be 34-35% zoom level.

At 100% zoom level, the document is much larger than the screen and the red box shows you where you are in that document. It's the part visible on your workspace. You just drag that red box to any area you want to see on your Document window.

You can detach and resize the Navigator panel if you need it to be larger or in a more convenient location on your screen.

In Photoshop, 100% has nothing to do with size. That's not what it means or refers to.

!00% means that one image pixel is represented by exactly one screen pixel. In other words, you're seeing every image pixel faithfully reproduced on screen.

The actual size on screen will be entirely dependent on the screen resolution. On a 4K/retina display, for instance, it will appear much smaller than on a traditional HD-class display.

Yes, it can be a bit confusing if you're used to vector-based applications like Illustrator or InDesign, where 100% does indeed refer to size. But not in Photoshop.