Final answer:
Based on Nigeria's age structure diagram from 2019, the population size in the country will be much larger in the future due to a pyramidal age structure and high birth rates.
Step-by-step explanation:
Based on Nigeria's age structure diagram from 2019, which shows a pyramidal structure with a preponderance of younger individuals, it can be concluded that the population size will be much larger in the future. This pattern is often observed in underdeveloped countries with high birth rates and less-than-optimal living conditions, where individuals do not live to old age. Therefore, it is likely that Nigeria's population will continue to grow rapidly, resulting in a much larger population size.