the assumption is you do not come up with your own words, and pick words at random from the whole dictionary.

I'm not sure I agree with that assumption, as the entire purpose of a passphrase of words rather than a password of random characters is that the passphrase should be easier to remember. If you're randomly picking words like 'gargarize-youster-noctivagant-axilla', it's not exactly accomplishing that purpose very well. It's also a huge PITA to type in, which based on my experience in the IAM space, is an immediate dealbreaker.

    $ egrep '^[a-z]{4,10}$' /usr/share/dict/words | wc
      50768   50768  433477
    $ for i in `seq 5`; do egrep '^[a-z]{4,10}$' /usr/share/dict/words | shuf -n 4 | xargs; done
    droned engraves developer manoeuvre
    lifeforms lurked pursuing subjugated
    hooligans underplay sudden command
    quartettes soapbox blacklist pigtails
    roughening chefs mortals earthy

In my experience, things like that are both easier to remember and to type than things like fa#klwgjl5235 - I type sequences of English words far more often than I type anything else.

I’d rather pick from obscure words I know than at random. In my case the words might lean tech/business/news/sports, but I’m sure I could come up with a good list. It might be interesting to try and generate passwords from a corpus of email and/or browsing history... assuming you blacklist sensitive subjects.

I let my password manager pick words for me, and I keep hitting refresh until I get one that I think I'm likely to get the spelling correct when needed.

1Password just gave me this: land convolve witchery bequest

Having said that, since I use 1Password, these are rare and almost exclusively used for things where I need very-short-term memorable passphrases for things that won't let me copy/paste from 1Password (like my Apple ID or the passwords my bank ask me for over the phone...) Everything else just gets 25 random chars (or the maximum number of chars the input will allow).

If i used that model i am pretty sure there would be some kind of proper noun or fantasy novel reference, meaning the Dictionary would need to be pretty extensive.

And - of course - Randall has us covered here too:

https://xkcd.com/1133/

A 1000 word vocabulary is quite understandable, though a little awkward.

A random word might be as few at 10 bits of entropy. If a person is picking them out of their head, I'd bet it's unlikely to be as many as 12 or 13 bits. Most of the words we "know" aren't ones that come to mind when we're "randomly choosing words"...