Well, I think that size is the number of bytes. Each character is stored on a char (1 byte) and "Hello world!" is indeed made of 13 characters in C. The size of the cstring is indeed 13 bytes.
Now, nitems is probably the number of arguments with `echo` counting as one (similar as argc for command line parameters). I am not sure, any confirmation (/infirmation) of this would be appreciated.
