Rendering on cluster - improving render time?

Hi,
I have finalized my animation and will now render.
What is the best way to optimize the full potential of a cluster?

So far I have managed to only utilize 1/4 of the computer power, and I do not understand how I can improve it.
Will it help to decrease the tiles size?
The animation is fairly simple, but the object is big and complex.
Any tips?

K