What organ does not change size from birth to death?
No boss its not like what you are saying, Eye's will remain same in size till to death from birth, this is what natures beauty.instead of this all organs will change there size. I don't think there is any body part or organ in humans that remains the same from birth to adulthood. There must be some form of changes if that body part or organ is normal.
Under what conditions might the per capita birth and death rates not be enough to accurately predict future human population size?
an increase of birth rate (the number of people giving birth) and a decrease in death rate (the number of people dying) can lead a huge population change in the fact that the pop will become larger. an example is India of which has a high birth rate and as a result in 30 years will be the country with a highest pop overtaking china