The dimensions of the fuselage are controlled by the number of passengers and the seating arrangement. A thin fuselage will have less profile drag than an equivalent wetted area fatter fuselage. The thinner fuselage will weigh more because of additional structural requirements. The engines provide thrust which is necessary for forward movement. Each provides a specified amount of thrust in cruise.
Engine efficiency varies from engine to engine.
The number of engines determines the total thrust available. The total thrust must be large enough to get the aircraft off the ground within the maximum takeoff distance. The thrust must also be enough to overcome the drag generated by the aircraft. Excess thrust will cause more fuel to be required. The role of the wing is to provide lift for the aircraft.