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Ian,
Having built my own obs I too had this problem, how big is big enough.
Unfortunately I am in clay country so I opted for a 1 cubic meter pier foundation using reo bars as reinforcement.
Also made the bottom of the pier hole larger than the top, so its a square cone shape, giving it a much larger footprint at the bottom.
I have a 2m High, 260mm OD 1 inch wall thick pier attached to this foundation and after two years it has moved.
Just recently with all this rain we have had in Adelaide the pier has slightly shifted. At the top of the pier plate that equated to nearly 1 degree shift.
If you have clay, and you dont want it to move, there is really nothing you can do. Does not matter how big you go you will get movement.
Make sure you have enough adjustemnt at the top of you pier to counter act any movement and you should be OK.
Go as big and as deep as you can, that is my advise.
Mark
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