Size of a nation’s economy is commonly expressed as:

Economists use various metrics to quantify and compare the economic activity and health of nations. These measures provide insights into a country's production, income, and overall economic performance. The question asks for the most common way to express the size of a nation's economy.

• GDP (Gross Domestic Product) is the correct answer. GDP is the most widely recognized and utilized measure of a country's economic size. It represents the total monetary value of all finished goods and services produced within a country's geographical borders over a specific period, typically a year or a quarter. It provides a comprehensive snapshot of a nation's economic output.
• Per capita income measures the average income earned per person in a given area in a specified year. While it indicates the standard of living or economic well-being of the average citizen, it does not represent the overall size or total output of the economy.
• GNP (Gross National Product) measures the total value of goods and services produced by a country's residents, regardless of where they are located. While related to economic size, GDP is generally preferred for measuring the economic activity *within* a country's borders, making it the more common indicator for the 'size of a nation's economy'.
• CPI (Consumer Price Index) is a measure of the average change over time in the prices paid by urban consumers for a market basket of consumer goods and services. It is used to gauge inflation and the cost of living, not the overall size of the economy.