This is a good question. If we limit the family size, there will certainly be social issues when it comes to the elderly. Let me mention two of them. First, as the population gets older, they will need people to help them. The elderly cannot always take care of themselves. If the family gets smaller, then who will help them? One child, for instance, cannot help two parents and take care of their own lives.
Second, If we say the solution is the government, then we would need tax payers to pay for the bill. But if we limit the size of the population, then we will not be able to pay for the social services. There will be a swelling elderly population that does not work and a dwindling young population that needs to support the elderly. In short, there will be big issues if we limit family size.