Oh, that kind of substrate independence. In Dennett's story, an elaborate thought experiment has been constructed to make substrate independence possible. In the real world, your use of "I" is heavily fraught with substrate implications, and you know pretty well which physical system you are. Your "I" got its sense from the self-locating behavior and experiences of that physical system, plus observations of similar systems, i.e. other English speakers.

If we do a Sleeping Beauty on you but take away a few neurons from some of your successors and add some to other successors, the sizes of their heads doesn't change the number of causal nexuses, which is the number of humans. Head size might matter insofar as it makes their experiences better or worse, richer or thinner. (Anthropic decision-making - which seems not to concern you here, but I like to keep it in mind, because some anthropic "puzzles" are thus helped.)