To get an idea of the size of your English vocabulary have a go at the test on this site. It takes about five minutes or so and is anonymous, although the researchers would welcome your telling them your year of birth and gender, as well as saying whether English is your native language (no need to register or give other details).
You just tick a word if you know at least one meaning for it - you don't tick it if you've seen the word but don't know what it means. Words start off very easy eg like, work, enough, but then get more difficult, then very difficult such as tattermedalion, pule and hypnopompic.
The project is part of an independent American-Brazilian project and the researchers claim it works for everyone -- native speakers, non-native speakers, children, or college professors.
The maximum score is 45,000 words. Some linguists would say that many people know more than that.
You don't need to look up the words you don't know in a dictionary; the site has links to the definitions of hard words in the list, those found fewer than 3 times per hundred million words. You might not find all of these words hard; it depends on your interests and what you read -- malapropism is among them, as are uxoricide, tenebrous, bruit, embonpoint and deracinate, which don't strike me as being all that difficult, although I suppose it helps if you know Latin and French.