Unfortunately CPYTOPCD is limted to 2GB in size or less. The following link explains this a bit more.
The general answer is “You can’t.”
The somewhat more specific answer is not to use CPYTOPCD. That command has been effectively obsolete for ten years or so and shouldn’t be used. It only works with the /QDLS file system (also effectively obsolete). Use CPYTOSTMF or CPYTOIMPF instead, and copy to a different file system such as /root.