Cornwall displaced 9,850 long tons (10,010 t) at standard load and 13,520 long tons (13,740 t) at deep load. The ship had an overall length of 630 feet (192.0 m), a beam of 68 feet 5 inches (20.9 m) and a draught of 20 feet 6 inches (6.2 m). She was powered by Parsons geared steam turbines, driving four shafts, which developed a total of 80,000 shaft horsepower (60,000 kW) and gave a maximum speed of 31.5 knots (58.3 km/h; 36.2 mph). Steam for the turbines was provided by eight Admiralty 3-drum boilers. Cornwall carried a maximum of 3,425 long tons (3,480 t) of fuel oil that gave her a range of 13,300 nautical miles (24,600 km; 15,300 mi) at 12 knots (22 km/h; 14 mph). The ship's complement was 784 officers and men.
The ship mounted eight 50-calibre 8-inch (203 mm) guns in four twin gun turrets. Her secondary armament consisted of four QF 4-inch (102 mm) Mk V anti-aircraft (AA) guns in single mounts. Cornwall mounted four single 2-pounder (40 mm) light AA guns ("pom-poms"). The ship carried two quadruple torpedo tube above-water mounts for 21-inch (533 mm) torpedoes.
Cornwall was only lightly protected with little more than a single inch of plating protecting vital machinery. Her magazines were the exception and were protected by 2–4.375 inches (50.8–111.1 mm) of armour. Space and weight was reserved for one aircraft catapult and its seaplane, but they were not fitted until after she was completed.