To measure the happiness of people is not a reason we might want to measure the size of the economy.
Measuring the size of the economy is important for various reasons, including tracking the magnitude of recessions, estimating the productive capacity of the country in case of war, and comparing living standards in different countries. However, measuring the happiness of people is not a direct reason for measuring the size of the economy, although some economists might argue that a larger economy generally leads to higher living standards and, therefore, greater happiness.