It depends entirely on the shed construction. With some you want the base to
be the exact size of the frame, with the cladding (shiplap or whatever)
overlapping the base - in which case the base needs to stand high enough to
stop water splash problems.
(Often there is minimal headroom in standard sheds, and a solution is to lay
a course of bricks on which to stand the shed)
If your slab is bigger than the shed you need to gaurd against rain falling
onto it and running inside
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