It estimated 74k words for me, but I feel this might be inflated; much of the time when I didn't know the answer - I could vibe guess it just as you did it. The distractor answers weren't convincing enough. For starters, when an answer was based on deconstructing the word into common English words, that ruled it out. After all, if it was, then it wouldn't have been obscure.
A tangent: writing distractors for multiple choice questions is hard. From the exams I know (excluding those whose nature precludes it, such as based on calculation or rote memorization) the only that does this brutally well is LEK (Polish medical graduate exam). It's nigh impossible to vibe guess it at more than random chance for someone outside the field.
