I took a vocabulary size test at testyourvocab.com. That test is for passive vocabulary. It shows a list of words and you're supposed to check a box after each word if you know at least one of its meanings. Based on the first list, the algorithm narrows down your probable vocabulary size to a band and gives another list of words of appropriate frequency to give a more precise estimate.
My result was 25,500 words. Most adult native speakers who've taken the test are between 20,000 and 35,000 words. The test has a 40,000 word ceiling.
There is an interesting graph on the website showing the distribution of vocabulary sizes among those test takers who report being native speakers. My result is equivalent to the median of 26-year-old native speakers. I seem to be about 20 years behind people around my age who speak English as their first language.