Regarding the chord issue, no expert here, but my understanding of ground effect is that it primarily results from interfering with the wing tip vortex. It reduces the loss of lift that ordinarily occurs at wing tips because of high pressure air on the bottom spilling over to the top of the wing at the wing tip, which creates the vortex.

Soaring gliders have high aspect ratio wings in order to minimize the proportion of the wing that is affected by this loss, so conversely, a low aspect ratio wing (wide chord in relation to span) will lose proportionally more of its lift to wing tip losses.

If the ground effect explanation above is correct, then obviously ground effect will will be greater on a low aspect ratio wing than on a high aspect ratio wing, because the low aspect ratio wing loses more to wing tip vortices, so preventing that loss should have a proportionally greater effect. The longer aspect ratio wings will still glide best, but the improvement from ground effect should be greater with a low aspect ratio. Anyone have a Slow Poke 40? That should have a pretty poor glide that improves noticeably close to the ground.

I'm not sure if that addresses the original question. As to models experiencing ground effect, like several others, I have witnessed this effect on some of my own models.

Jim