Could the upper slide plate be smaller, sized to support the load, and the lower slide plate sized larger to accommodate the movement of the upper plate?

Yes, but it is standard forthe upper slide plate to be larger and the lower plate to be smaller which helps tokeep them clean.

PTFE, 25% Glass Filled Slide Plates Bonded to Stainless Steel Backing Plates
PTFE, 25% Glass Filled Slide Plates Bonded to Stainless Steel Backing Plates
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