[Courtesy of Jeff Reid [jeffreid "at" pacbell.net], August 1999]
On full scale gliders, a larger aspect ratio improves the lift to drag ratio. The limiting factor is how to build a reasonably strong long narrow wing. On faster powered planes, parasitic drag is more of a factor than induced drag, so these use shorter wing spans.
With 3 meter or longer wing spans, high aspect ratio wings are still very efficient, but as the wing span or speed is reduced, aspect ratio runs into a different aerodynamic based limit. Smaller and/or slower gliders need some minimum amount of "chord" (front to back distance) in a wing to provide a reasonable amount of lift without an associated excessive angle of attack (at the slower speeds they fly at). Once Reynolds numbers get near or below 70,000, then the aspect ratio for the best lift to drag ratio will be smaller.
A formula for Reynolds Number (Rn): = wing chord (inches) x speed (mph) x 780 (conversion factor)
Note that its the wing chord and not the wing span used here.
Look at the aspect ratio on the high end HLG gliders, which have about a 5 foot wing span. Most will have around 1/2 foot of chord, for an aspect ratio of around 10. This is probably the most efficient aspect ratio for planes in this class.