For others who want to test this git command (if they do not knew them before) just execute this on CVSAnaly repo:

$ git cat-file -s 12636db:tests/file_path_test.py
> 2638
$ git cat-file -s HEAD:tests/file_path_test.py
> 4679

This are the two versions of the command.
The size is measured in bytes. See http://git-scm.com/docs/git-cat-file