design for plywood under slab

If you are talking about a structural slab, the plywood must be sized to accommodate the dead weight of concrete plus about forty psf construction load (workers placing the concrete). The thickness of plywood will vary according to the slab thickness and the spacing of supports. For thin slabs, you will probably use 3/4" plywood and space the supports appropriately for the load being carried. For thicker slabs you may elect to use a heavier plywood.

There are tables for concrete retaining walls and plenty of information available for other types of retaining walls. I think you must be a little more specific about your design requirements before we can respond to your question.