The depth is usually governed by local ground conditions. Going on the basis that you are digging virgin soil (as opposed to made up ground), and that you are through the growing matter, then dig a bit further for the depth of concrete.
So lets say you are down to firm, non-vegetative ground at 600mm deep. dig out say another 150mm.
Lay a minimum of 200mm thick concrete, 450mm wide. The concrete can be laid thicker to reduce footing masonry as this can save money on skilled labour. But if you are doing the block work yourself then fill ya boots.
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