Arlington, Texas is 4.3 times larger than San Marino.
Area: 61 km2 World Factbook
Population: 32,742 (July 2014) World Factbook
San Marino, officially the Republic of San Marino and also known as the Most Serene Republic of San Marino, is an enclaved microstate surrounded by Italy, situated on the Italian Peninsula on the north-eastern side of the Apennine Mountains. Wikipedia
Area: 265 km2 calculated
Arlington is a city in the U.S. state of Texas, within the Dallas–Fort Worth–Arlington metropolitan area and Tarrant County. According to the U.S. Wikipedia