The Oxford Advanced Learner's dictionary has a 'defining vocabulary' of about 3500 words. They claim most definitions are limited to the use of those words and proper names. So, that 6000-8000 looks believable.

Even writers of high-brow literature use a much smaller vocabulary than the one they can read. Probably more than the newspaper, but not likely 35,000 words.

For non-native speakers, I would guess that there are quickly diminishing returns past about 10,000 words. Much more important is how well you're able to use those first 10,000 (or maybe even 5,000). It's one thing to recognize a word when reading it, and another to be able to use it in a natural way, choosing words with the right shade of meaning, and avoiding awkward constructions or unintended connotations.

Googling says that Shakespeare used 31534 words , of which 14376 appear only once, 4343 twice: http://biomet.oxfordjournals.org/content/63/3/435.abstract

Not a modern corpus, but reasonable.

Strong's Exhaustive Concordance of the Bible (that would be the King James version) lists 8674 Hebrew root words for the pentateuch, and 5624 Greek root words for the New Testament.

Wikipedia suggests that full literacy in Chinese requires knowledge of about 4000 characters.

Note that in Chinese, character is not the same as word.

Most modern Mandarin words are bisyllabic, and represented by the combination of two characters, and their meaning does not necessarily derive in a straightforward way from the meanings of their constituent characters (for example, the word for "thing" is composed of the characters for "East" and "West" in conjunction)

Of those unique words, how many did Shakespear invent?

Making up new words is an easy way to boost vocabulary.

Definitely. I would have liked the test to be "would you be comfortable using this word in a sentence? (if you had to)" Even as a native speaker there are times when I know the definition of a word, but opt for a simpler word because I don't know all of the implications of using it.

And even if you were, you have to make sure that your listeners understand you, too.

Right, unless you want to ruin your point entirely by stopping to define words. Communication relies on shared vocabularies, not just one person's vocabulary.

There are plenty of words I know that I generally avoid in conversation for practical reasons. Entertainment reasons, though... I caught my wife teaching our not-yet-two-years-old daughter to use the word "etiolated" when referring to a green bean that was wilted and yellowish.