Felix was 3" 11" and a former circus acrobat.
I can’t recall ever seeing the person inside the Cousin Itt costume. It was a cool costume and memorable character.
I remember Felix better as Twiki the robot on Buck Rogers.
RIP
Rest in peace, Itt guy!
He was a Talosian in the first Star Trek pilot.
RIP.
All I have to say is this:
Here is his IMDb entry. Reading it, I said “He played THAT character?” more than once.
Felix Silla - IMDb
He was ubiquitous in film and TV for any diminutive character. One character not really publicized that he played was Gollum in Ralph Bakshi’s 1978 Lord of the Rings. Sharon Baird played Frodo and Billy Barty played Sam (and Bilbo).
Sounds like gibberish sped up. 
But I never was good with accents.