Estimate your English vocabulary size 15 years ago
I took the opposite approach.
I checked all the boxes on the first page but one, "vibrissae" (roughly, whiskers), and saw even more boxes on the second page and groaned. So I punted, and went back to the first page, reloaded it, and checked only one box: vibrissae.
After finding 17 words on the second page, I left them all blank, following the same methodology. My vocabulary size was estimated to be 20 words.
From this, I deduced that my total vocabulary size was all the words ever known to any English speaker anywhere, anytime - minus 20.
This made me very pleased with myself, even though I knew my assumptions were pretty terrible.
(Interesting that the spell check in my browser doesn't recognize vibrissae.)