Bore and Stroke
The bore and stroke of the piston sets the swept volume.
The ratio of bore to stroke is a good indicator of the engine characteristic such as how high it will rev and basic fuel economy. More simply it also defines some of the basic dimensions of the engine such as height and length.
Bore versus stroke for a number of production engines. [Ref: Shayler]
80mm bore and 80mm stroke would result in a displacement of 402cc. With the bore equal to the stroke this is sometimes referred to as a "square engine".
If you reduce the bore and stroke together then the displacement reduces. Halving the bore and stroke reduces the displacement to 50cc or 1/8th of the displacement.
A stroke that is greater than the bore is known as a "long stroke" engine. This type of engine will have increased torque and good fuel economy but will not be able to run at high speed.
If the bore is greater than the stroke then the engine is known as "over square". This is type of design is used in high performance engines as used in racing cars.
P. J. Shayler, J. P. Chick and D. Eade, "A Method of Predicting Brake Specific Fuel Consumption Maps", SAE 1999-01-0556