The output of a generator is dependent on the amount of torque that is applied to it and the speed at which it rotates. The generator needs to be sized appropriately according to the size of the rotor – you could but a 3000 kW generator onto a 50 m rotor diameter but it wouldn't be very efficient. a 500 kW generator would be better sized to a 50 m rotor, just as a 3000 kW generator is appropriate for a much larger 100 m rotor diameter.
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Here is a great site that covers most everything you'd need to know about wind energy.
http://www.windpower.org/en/tour.htm

Hydro power (e.g. underwater turbines) follows the same concept as wind energy, only the medium (water) is much more dense and there are a lot more complications with relation to the environment, fish, corrosion, and maintenance to the turbine.

Good luck!

In the hydroelectric world, the two factors that determine the potential amount of power to be generated are the design of the turbine and head/flow of the water. The greater the head/flow, the more power that can be produced. It is not unusual to see individual units that are several hundred megawatts in rating in hydroelectric plants.

Fundamentally, the same factors are present in the case of wind. However, wind tends to be limited in its availablity. Therefore, the issue becomes that the diameter of the turbine becomes the limiting factor in determining the rating that is possible. The largest wind turbines today are around 5-6MW, and it is unlikely that we will see anything much larger than perhaps 8-10MW in our lifetime.
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In a theoretical world w/o wind drag, friction, etc, the two solutions should behave equally. However, with many generators, there is the rotational friction (as the motional part spins around and around), air resistance as the moving parts move, etc. These forces take away some of the overall output, so minimizing these forces will maximize output. Having one big generator is more efficient because a smaller percentage of it's energy is lost to the world.

Do you know what kinetic energy is? it is the equation used to describe the amount of energy a moving object has. it is:
kinetic energy = 1/2mv^2
where m = mass and v = velocity.

It is difficult to do anything useful w/ generators w/o calculus but just know that the kinetic energy of the moving water (or wind) get's directly converted into electrical energy when it cycles through the generator.

By looking at that equation you can see that if the water (wind) moves faster, the kinetic energy increases, which means the generator output increases.

It takes more energy to move bigger sized revolutions, so by increasing the size of a revolution it would only slow down.

If you sped up or slowed down the revolutions (which would happen with greater velocity of water or wind), the kinetic energy would increase so the output would increase.
References :
bioengineer