I agree with most of your points. Size, IQ, and environment don't make someone less human. But I think there are some other differences you did not highlight. For one, nearly every animal can create a fertilized egg and at their core they're fairly indistinguishable. The difference is in a small part of DNA (of which humans share roughly 97 percent with other animals). So while two humans could create an embryo, does that make it human when two different animals could create an embryo as well? Regardless of what it becomes, it's still an embryo. When does that embryo become a member of a species? Does the fact that every other animal make embryos provide sufficient reason to conclude it is a human life?