How to change the font size of Open/Save dialogs?

On Sierra ... my Open/Save dialog file listing are displaying in a larger-than-normal font size.

How do I change this?

It depends on which application is showing the larger text as it may have it's own view settings. For instance, your Finder window has view settings (accessible from CMD-J or View->View Settings) where you can change the font of the view window to produce exactly the results you are seeing (the default is 12), whereas in Mail you can configure this from the Fonts & Colors section of Mail Preferences. I know in some programs I've used that those settings are in the data stored in Application Preferences in the Library.

If nothing else works and it's system-wide there isn't really a systemwide way to change fonts in macOS but there are some tools out there to help. You could try TinkerTool to see if that is helpful for you, there is a specific tool to adjust macOS system fonts:

In Mojave, at least, Font sizes in Open/Save Dialog Boxes are stored in ~/Library/Preferences/com.apple.finder.plist

The settings for Column View are in FK_StandardViewSettings:ExtendedListViewSettingsV2

The settings for Icon View are in FK_DefaultIconViewSettings

The settings for List View are in FK_DefaultListViewSettingsV2

Relaunch Finder after making changes by holding alt and control and clicking on Finder's icon in the dock and clicking Relaunch.