I understand your concern. I am a small person myself, but I am not as fit as I should be.
I mainly fast for spiritual reason to not only to "reset" my body mentally and emotionally, but physically as well. So I know I have to watch for the tell tale signs that my body needs to stop my fast due to my figure.
I try not to worry too much about it because my body will tell me. All I have to do is listen to it.