This has been a big issue when using the abbreviated Finder dialog boxes when opening or saving a file in a Mac app. The icons are quite tiny and many of us want that visual feedback without having to tap the Spacebar for a quick look and and as most here know, the dialogs lack the icon size slider that's standard in the Finder. So I just tried a fix that even works in Sierra thanks to an older freeware program called "Pref Setter." You get it here: http://www.nightproductions.net/prefsetter.html It works even in Sierra. Since some folders are hidden by default, use the Go > Go to Folder option in the Finder menu and enter ~/Library/Preferences/com.apple.finder.plist in the field. Open com.apple.finder.plist in Pref Setter. You should see the FK_DefaulticonViewSettings key as shown below, and if you click the tiny triangle before it, you should see the iconSize entry. I changed mine to 256, pressed Enter, then closed and saved the file. Then I Restarted my Mac to see the big thumbs in the Open dialog. You can also try the "Relaunch Finder" button if you want to skip the Restart. (Nervous? make a back up of the original plist.) I hope that Key is still there. I don't know if it is removed for some.