Prism maintains a file called .prism_defaults in your home directory. In it, Prism keeps:
Changes you make to Prism via the Customize utility
Changes you make to the tear-off region
Changes you make to the size of the panes within the main Prism window
Do not attempt to edit this file; make all changes to it through Prism itself. If you remove this file, you get the default configuration the next time you start Prism.