This would be a very useful feature to work with a folder of image files, but unfortunately, it is not possible to display the pixel size of an image in the list view of nautilus. At most, you can check the pixel size in the file properties, and even then, it works quite clumsy if you need to inspect multiple files. You can quickly summon the properties dialog with the shortcut key alt+Enter, but you need to do that for each and every file and it always defaults on the "Basic" tab, so each time again you need to move to the "Image" tab...
"Files" developers clearly did not give this use case a good thought. You will need another tool. The default image viewer, Eye of Gnome, allows to navigate through pictures while seeing the image info. For that, set both the "Side pane" and "Gallery" to show. You see the image and its properties, and with the arrow keys, you navigate through the different images in the current folder. This actually works quite well, because Eye of Gnome remembers the settings. Next time you click an image file in Files, Eye of Gnome will open the same way with the Gallery view focuses, so you can immediately browse through your files with the arrow keys. Of course, while in Eye of Gnome, file management actions are very limited.
