Here's a splendid site:
http://testyourvocab.com/
Just indicate the words for which you can give a definition, and it'll estimate (+/- 10%) how big your English vocabulary is.
The first page is to get a very rough estimate of your vocabulary, the second page is specific to you, and allows the site to get a really good estimate.
My vocabulary is around 36,000 words, according to the site.
They have some nice data on vocabulary growth with age.