What aspect ratio is best for Wind Tunnel Testing an aerofoil with Small Additives? Vortex generators will be 3D printed on a panel, which is then to be slotted onto a foam aerofoil to be tested in the wind tunnel for performance and flow physics.

What I want to know is which aspect ratio to use?

Rectangular wing, need a chord length and span.

Max wing span is 700mm.
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.