Since Intellij is now a signed application it's no longer recommended to edit Info.plist. So now one should copy the default idea.vmoptions and edit the copy.
`cp /Applications/IntelliJ\ IDEA\ 12.app/bin/idea.vmoptions ~/Library/Preferences/IntelliJIdea12/idea.vmoptions`
Edit the copy and restart the IDE.
Note: I believe that the above is only valid for the Ultimate Edition.