Frankfurt is the largest city in the state of Hesse in the western part of Germany. Site. Frankfurt is located on both sides of the river Main, south-east of the Taunus mountain range. The southern part of the city contains the Frankfurt City Forest, Germany's largest city forest. The city area is 248.31 km 2 (95.87 sq mi) and extends over 23.4 km (14.54 mi) east to west and 23.3 km (14.48 mi) north to south.