What aspect ratio is best for Wind Tunnel Testing an aerofoil with Small Additives?
What is it you want to test?

Do you want to test the 2D characteristics of an airfoil? Then make it the width of the windtunnel. The wind tunnel walls act like mirrors, so you will have the behaviour of an infinite wing.

If you want to test a 3D wing on a model, you need to use coherent scale. Use the same scale for the fuselage/wing/empennage.