The ground effect is present irrespective of the size of the aircraft. Problem is that very seldom do we fly low enough with our model aircraft to notice it. Ground effect depends on wing span as well as chord and in a somewhat simplified model the effect can be separated into a span-dominated part and a chord-dominated part.
The span-dominated part of the ground effect is associated with a decrease of the induced drag of the wing. What happens is that that the wing-tip vortices resulting from the pressure difference between the upper and lower surfaces are displaced outwards, away from the wing. This results in a decrease of the induced drag of the wing similar to what would happen if the aspect ratio of the wing is increased. In most cases the drag reduction is accompanied with an inrease of the slope of the Cl vs. alpha curve. Experimental as well as theoretical work has shown that as the wing approaches ground, this effects is gradually introduced. The effect becomes significant when the height above ground is less than approximately one wingspan. In order to obtain a 20% decrease in induced drag the height above ground of the wing must be appoximately 1/3 of the wingspan or less.
The chord-dominated part of the ground effect is associated with a change of the maximum lift coefficient of the wing. For flat bottomed airfoils or airfoils with some camber, the maximum Cl may increase somewhat if the flying altitude is small compared to the chord of the wing. I don't have figures at hand but I may be able to dig some experimetal figures out from my archive, given enough time. From memory I recall that for this effect to be readily noticed, the height of the wing above the ground should be less than one wing chord. If the airfoil has very little camber, i.e., approaches a symmetrical shape, and/or the angle of attack is small the maximum lift coefficient may be reduced. In these cases the wing lower surfce and the ground casues a venturi effect, reducing the the dynamic pressure on the lower surface of the wing.

Edit: Tried to improve the spelling and grammer somewhat.