Change the font size of Safari's Web Inspector
Thanks for the answers above. As an update, in Mavericks 10.9, the file is still Main.css and can now be found at the following path:
/System/Library/PrivateFrameworks/WebInspectorUI.framework/Versions/A/Resources

You may also find the file using following terminal command:
find /System/Library/PrivateFrameworks -name Main.css