If I remember correctly, I used this article when I needed to uninstall Xcode:
http://macdevelopertips.com/xcode/how-to-uninstall-xcode.html
You open Terminal application and type:
sudo /Developer/Library/uninstall-devtools –mode=all
It will ask you for the password.
Such uninstall will give you about 2GB (again, if I remember this number correctly).