Where can I get a writing sample?

What metrics are available to determine how advanced a given writer's vocabulary is? How large a writing sample is necessary to get a good answer with such algorithms?

• We could apply such algorithms to the questions and answers of quora users and thus answer the question: I am thinking that this could be done using a huge set of writing samples to determine the rarity of each word and then studying the rarity of words used in the writing of the given author. I can imagine a huge number of ways of doing this. Which ways make the most sense and why?

• Answer:

  http://testyourvocab.com/ does something very similar, but as as an exam type thing rather than corpus analysis. One might argue that it is more accurate due to people sometimes using words in their writing which they don't fully know.