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I went for a full meter square and meter deep. I am in an area which is fairly rocky but it is mostly floaters and the rest is fairly heavy clay, so I wanted a good solid lump of weight under there. I ran in to a rock right at the bottom so I pinned it to that as Glen suggested above. Might as well make use of the mass if it is there.
In your own case if you run in to sandstone you probably need to work out if it is a big solid lump of it or something smaller to decide if the best bet is to break it out and keep going, or pin to it and make it part of the block. When I was doing that, I have a big hammer drill so I plunged the hammer bit in to it's full depth, I figured if it broke out of the other side I should break it up but as it was still in rock at full depth for the bit I assumed it is a reasonable size and pinned to it.
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