* guix/scripts/size.scm: New file.
* Makefile.am (MODULES): Add it.
(SCM_TESTS): Add tests/size.scm.
* doc.am (SUBCOMMANDS): Add 'size'.
* po/guix/POTFILES.in: Add guix/scripts/size.scm.
* tests/size.scm: New file.
* doc/guix.texi (Packages with Multiple Outputs): Add xref to "Invoking guix
size".
(Invoking guix size): New node.
(Invoking guix gc): Add index for "closure" and xref to the above.
* doc/contributing.texi (Submitting Patches): Use @enumerate for the check
list. Add item about 'guix size'.
