It's 6.4 liters - referring to the engine capacity/volume or engine displacement. This is the combined capacity of all the engine’s cylinders.
An engine’s size is otherwise referred to as its ‘capacity’ or ‘displacement’. Without getting too complicated, it’s a measurement of how much space the engine’s pistons operate in. A bigger number means each piston is able to push more air and fuel through the car’s engine every time it moves.
The number is expressed in cubic centimeters (cc) and on engines with a capacity or displacement greater than 1,000cc (one liter) it’s usually rounded up to the nearest tenth of a liter. As a rule of thumb, the bigger this number is, the more power you can expect the engine to produce.
Each piston moves up and down inside its cylinder – the space it moves through is called the combustion chamber. This is where fuel is burnt by the engine to produce power – the explosive force caused by the fuel burning causes the pistons to move.
If a car is described, for example, as a 2.0-litre, this means the engine’s combustion chambers allow roughly 2,000cc (two liters) of fuel and air to be burnt with every revolution the engine turns.
For more info about engine sizes, refer to the following sources below:
Car Buyer - What do engine sizes actually mean?
Crank It - Engine Capacity (CC): What is Engine Capacity, Engine Volume or Engine Displacement?