It seems more to be related to Windows functionality as the PSD thumbnails outside Photoshop resize/view as per the OS.
I use a Mac...hearing from others that this is also a problem with them..
But I'm using photoshop on a Mac and want to see the thumbnails larger in the open screen in Photoshop. ..
/usr/libexec/PlistBuddy -c 'set FK_StandardViewSettings:IconViewSettings:iconSize 48' ~/Library/Preferences/com.apple.finder.plist
48is the new icon size, and restart to apply the changes. Just logging out and back in does not seem to apply the changes.
FK_StandardViewSettingssettings are used by FinderKit and file dialogs. They are not changed when you press the "Use as Defaults" button from Finder's view options.
If you run the command on a new installation and there is an error like
Set: Entry, "FK_StandardViewSettings:IconViewSettings:iconSize", Does Not Exist, open a file dialog once in some application first.
Is this command supposed to be entered in Terminal.app? I can't see a Run command.
There's a different incantation for Yosemite and El Capitan, but seems to not work in Sierra.
/usr/libexec/PlistBuddy -c 'set FK_DefaultIconViewSettings:iconSize 192' ~/Library/Preferences/com.apple.finder.plist
As a workaround Spacebar can Quicklook the icon and you can use the cursor keys to move from file to file in the dialog in Quicklook mode.
But somehow Sierra inherited my El Cap icon settings.