Ghent University offers an online vocabulary test designed to provide a valid estimate of your English vocabulary size.
The test does not require you to define or spell words. Instead you are asked to determine whether an entry ("glyph," "moktam," "macrophage," "wookel" etc) is or is not a word in the English language.
There seemed to be about a hundred words in the test, which you can proceed through at your own pace, and you can retake it if you wish (with a different group of words on the retest). At the end you can review your errors and see the definitions of the words you missed.
I don't know how to interpret my results (second trial shown). I correctly recognized 94% of their offered words, but I almost certainly don't know 94% of all the words in the English language. Or maybe I do. Perhaps someone can drill down into their methodology.