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I assume you're asking about the turbofans on passenger airliners, so that's what I'll describe. First, look up "high-bypass turbofans" for a picture.
The actual "engine" occupies only about the center third of the cowling you see hanging below the wing, the rest is air...
The engine provides the power to spin the fan (the blades you see from the front). The fan then moves a much larger amount of air through the duct (cowl) than a propeller would spinning in the open air. This is crucial for high altitude operation because the density of the air at altitude is so much less than at sea level that it's almost impossible to move a sufficient WEIGHT (not volume) of air backward to provide enough thrust for propulsion.
The cowl or duct improves the effiency of the fan by preventing the air from spilling off the tips of the fan blades. If you look at the wingtips of long-range aircraft like the 747-400, you'll see "fins" (winglets) sticking up from the wingtips. They do exactly the same thing aerodynamically as a duct except that it's for lift, not thrust.
For another interesting technology, look up "unducted fan". I think Lear has started using them now.
Answer
This may be over-simplified, but jet engines are large because they have to produce a large amount of thrust due to the size of these passenger aircraft. During take-off, the turbo-fan engine can produce 50,000 lbs of thrust for each engine. That is tremendous compared to the older propellor technology.
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Where does one get information on airplane engines?
One can get information on airplane engines from the website of the manufacturers of airplane engines. There will be many different answers to questions one may have.
How many engines does a 1950 passenger airplane have?
There is no such large airplane so far and such large ones will be necessary and impossible
What helps an airplane overcome drag so it can thrust forward?
An airplane uses engines to generate thrust, which propels it forward. The engines work to overcome the drag force that resists the airplane's forward motion, allowing it to achieve the necessary airspeed for flight. This is necessary to create the lift that keeps the airplane airborne.
Who are the leading manufacturers of airplane engines?
There are many manufacturers of aeroplane or airplane engines. The leading manufacturers of airplane, aeroplane, or jet engines are Boeing, General Electric, and Pratt & Whittney.
What is the noise of an airplane?
The noise of the airplane comes from the engines.
What is the distance of airplane?
Depends on the airplane size and engines used.
What provides thrust to a real airplane?
The thrust is an airplane is provided by the engines.
What is the difference between jet and airplane?
Any fixed-wing aircraft with an engine is an airplane. There are different kinds: piston-engine airplanes have piston engines driving propellers, (airplane engines are horizontally-opposed, like the engine in an old Volkswagen), turboprop airplanes have turbine engines with gearboxes that drive propellers, and jet airplanes have turbine engines that move the plane directly. So, factually, a jet is a type of engine on an airplane. However, over time, aircraft with jet engines have often been referred to as "jets."
What causes an airplane to accelerate?
An airplane accelerates due to the thrust generated by its engines. As the engines produce forward thrust, the aircraft gains speed. The thrust must overcome drag forces acting on the airplane to achieve acceleration.
What makes the airplane move?
The engines, driving propellers or jets, make the airplane move.
How much do airplane engines weigh?
Most people are surprised how much airplanes weigh, and engines too. Reciprocating engines typically weigh 1 pound per HP. Big jet engines weigh 0.2 pounds per HP or so and they typically weigh from one ton to several tons. Here a simulator for designing your own engine: http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.htm
What keeps an airplane moving forward?
An engine producing THRUST keeps an airplane moving forward. Types of engines used by airplanes include reciprocating engines, turbo prop engines, turbojet, and turbofan engines. Some manufacturers are developing electric engines and one bicycle racer powered a very lightweight airplane using pedal power connected to a propeller.
