I have file vault and a firmware password on my laptop. I have 13.14 gigs of space left on my drive after deleting a few things which should be enough.
this is on a 15 inch mid 2014 mac book pro.
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A system restart resolved the issue, you can install this beta with a firmware password and file vault enabled.
Just a heads up to anyone wanting to test the public beta. A number of applications that run system daemons do not work on 10.11. I am personally downgrading back to a stable release until more developers update their applications.