In the Menu Bar of the Finder go to View -> Show Preview Options, then you get a dialog where you can select which info should be shown in the preview.

Ok, this is crazy.

1. I open the dialog, "Dimensions" appears (it was already selected)

2. It fails to load the dimensions of a screenshot macOS itself took: https://imgur.com/ufs0UfZ

3. I close the dialog, the new fields disappear

To me this confirms that macOS is still a toy for Apple 20 years later.