I got 4 yards to be safe, but I think 3 was plenty. And I don't have the costume with me but LUCKY 4 U ! remember the exact dimensions! The tentacles are all 3 feet long and the head ended up being exactly 27"x27"x27". HOWEVER. IDK if you should use my dimensions, 'cause here is what i did and what would probably be best for you too (pics under cut):
I also wasn't sure about size, all I knew is that I wanted to be able to fit through an average doorway SO. I taped up paper to the door and traced myself on it, and then.... just drew a squiddle over myself abut the size that seemed right:
(anime close up)
And THEN you have a have a handy reference for when you're like "what's the diameter of the top of the dome again?" "i forgot how big the eyeholes should be" etc etc. and it's custom fit to you!! 27"^3 is kind of a weird dimension to work with so you should probably just find the dimension that fits yo personal cranium.
ALSO, bonus! I found a pic of my in-progress styro-dome if you wanted to see:
Anyway, good luck! also if you have any more questions feel free to be not-anon, it ok don't be shy! COME FORTH let's talk about SQUIDDLES