Some clever folks analysed Shakespeare’s writings and came up with the following conclusion:
This means that in addition the 31,534 words that Shakespeare knew and used, there were approximately 35,000 words that he knew but didn’t use. Thus, we can estimate that Shakespeare knew approximately 66,534 words.
Some other clever fellow says that the average English speaker possesses a vocabulary of 10,000-20,000 words, but only uses a fraction of that (somewhere around a sixth). [Source]
Why not take this test and see how many of the specially selected 100 words you know. It’s multiple choice (and no, the answer is never D) and self-grading (you have to count your correct answers to get your percentage). I scored 72%, which would have been much lower without my knowledge of Italian (because it is closer to Latin).
Go on, take the test!