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Mine is 350 x 350 x 750 deep. It would've been 1000 deep but I hit rock at 700mm so chipped into it about 50mm to key the footing to the bed rock and you can't get any better.
Depth is what you want in a pier footing rather than mass. The pressure holding the footing from the sides increases with the square of the depth, and you can have active soil layers above the bottom of the footing which can expand and shrink all they want, but the footing won't move. Same principle as foundation piles.
Even a huge massive footing on top of an active layer will move.
Al.
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