"Hello" is saved as "Hello\0", "\0" being the NULL terminator.
strlen() is used to get the length of an array of chars / string.
strlen() takes a pointer to a character, and walks the memory from this character on, looking for a NULL character. It counts the number of characters before it finds the NULL character.
sizeof() is used to get the actual size of any type of data in bytes.
Hence it would include the NULL terminator as well.
if(sizeof(string) <= strlen(string))
if-condition fails, so print 0.