Engine size is the total swept volume of the pistons of the engine. Generally the larger the engine the greatr the power and torque output. Turbo chargers improve the engines breathing efficiency by around 30 - 50%. Thus a 2 litre turbo might drive like a 3 litre without.

Fuel economy is affected by many factors as well as engine size. Look at HJs real mpg guide.

The above is a generalisation. One could write a post comparable to War And Peace on this subject.

... difference between engine sizes ....

They mean just what Cyd says - the total volume of the cylinders - usually 4, sometimes 6 or more, in larger cars - in which the fuel is burnt. Small cars get by with 1 to 1.6 litres, and the larger the capacity, the 'better' the acceleration and overtaking ability.

Economy has as much to do with your driving habits as anything else. Fuel consumption is affected by the weight and shape of the car as much as the engine size. A big engine in a small car can be surprisingly economical.

I must say it shows some bottle to register yourself as a geek and then ask a question the average 9 year old could answer.

Petrol or Diesel?

The larger the area of fuel/air mix which 'explodes' under the pressure of a minimum of two, but as many as 10, pistons, the more powerful the kick-back of said piston...drive shaft...wheels.

Size doesn't mean anything - it's what you do with it that counts, and how long it might last after many years of use.....