I just remembered this but I'm surprised that no one has mentioned it yet. It was CrateCruncher's post that triggered the foggy memory. He's right and as the spacing grows the effect of the plate reduces. At some point it won't matter how big the plate is the lift will be back to "normal"

Helicopters have their own unique version of ground effect. It results in a high degree of rotor wash recirculation. If someone checks on this they'll find diagrams with a torus or donut shaped vortex flow where the rotor wash is pushed down and out but then flows back towards the lower pressure region over the rotor disc and back into the flow. But without checking I can't remember if it adds to the lift as with a fixed wing airplane or detracts due to having a residual downwards flow component from the donut shaped vortex flow that occurs when in this ground effect. Actually it would appear to reduce it based on this test since as mentioned as the plate to heli distance grows the effect of the plate will lessen