they look great, and so does the cassava - they are both bigger than mine! I dont have any good answer to your question, but i suspect the answer may be more how long they have been growing rather than how big they are. I did dig up some small ones to move them, and although they had no roots to speak of (nothing harvestable anyway) they reestablished themselves really quickly. So if you dont get any real advice about this - my advice is to dig one up and have a look...it wont do any harm.
Sorry Addy, although we grow this, but never tried eating them.. the chooks like their leaves, and supposedly good for the compost too. I would be interested to find how how you would eat them though and photoes would be much appreciated ~
sorry - I missed this one too - was away on hols (I never want to advertise this in case someone decides to rob our house!)
I think you should leave the roots for two or three years to get a decent sized root for harvest - it's the pith in the centre that is eaten yes? so you'd need a fair size otherwise once you peel them you won't have anything much left..
but i don't know, i've never tried it and i've never read anything that talks about this :)