Just like with plywood, 8x4 is the standard size for a sheet.
You can get the thinner gauges on a roll, but it might not be strong enough
Where do you think you'll get your stone from? Can be very expensive.
The wall would only need to be retaining if built vertical and filled in behind, what about battering up the slope in dry stone, planting up between and pockets for colour and interest?
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