Born2Bake, when it comes to sizing baking stones, bigger is always better. It gives you the option for larger pizzas (which you will end up wanting to do) and it gives you a larger target for smaller ones. In your particular setup, that 18" dimension is pizza gold. You do NOT want to go smaller than 18". At the same time, though, an 18 x 23" steel plate will not allow air to flow from bottom to top, which has to happen. In order to allow for air flow and not jeopardize your 18" potential, the gaps have to happen on the 23" dimension- preferably at least an inch on each side.
Your goal should be an 18" x 18" x 1/2" steel plate. So far, we have yet to run across a member who's shelf couldn't handle the weight of the plate. Think about the weight of a Thanksgiving turkey. If you are feeling especially paranoid, run two flat steel bars from side wall to side wall (resting on the shelf lip) and put the plate on that. Angle iron (or even heavier weight angle aluminum) will also work, as long as it is pretty small and can fit in the opening between the two shelf lips.