How is a countries population and the size of its labor force related

If the population is larger then this will require the labor force to be larger also. people need jobs and there will be more services to provide

It's all about the cost of living in any country, the higher the cost of living in a first world country, US, would entail to higher amount of people working. More people, more dense of a population would lead to inflation of living in any given area, leading to a greater number of labor force.