A few more comments about essays.
My general rule about long essays is: if it's in your application, don't put it in your essay. In other words, your essays should ADD information and insight into who you are, not reemphasize what you've already told them in your academic or EC record.
Also, don't feel like you have to tackle a big or heavy, 'change the world' type of subject. I believe that 'smaller' topics are often much more effective in shining a light on what makes you unique--ie. why the school wants YOU.
One device that both my kids used for an essay was to combine and relate two apparently different subjects. For D1, it was baking and hiking, she described how a hike we took kept reminding her of various parts of the baking process. It was a narrative of how she viewed the world around her. D2 had an essay about playing Scrabble; it included a connection between the aesthetics of certain words and chemical structures, linking her fascination with words to her interest in science.