osx - How can I increase the size of thumbnails when opening files? - Super User
It's a Terminal command. Open Terminal, copy and paste this command
/usr/libexec/PlistBuddy -c 'set FK_DefaultIconViewSettings:iconSize 192' ~/Library/Preferences/com.apple.finder.plist
Press the Enter key and it should work immediately, especially for Yosemite.
Gene