The answer is that the similitude is governed by the non-dimensional numbers. You can scale the dimensions but you have to increase the velocity in the wind tunnel in such a way to get the same Re number. A further parameter, the Mach number can be required. Clearly the fluid must be the same to have the same viscosity.

You also need to check the dependence of your targeted quantity on some dimensionless numbers such as Reynolds number. It is usually very hard to match the Reynolds number (or some other dimensionless numbers) of wind tunnel experiments with those in real applications. Therefore, you can run experiments with increasing Reynolds numbers to determine how your targeted quantity may depend on it. To make an accurate prediction for real applications, ideally, you would like to see your targeted quantity become independent of Re when Re increases above a certain threshold.

You should take care of parameters like dynamic and geometric similarity for any wind tunnel tests. The main non-dimensional parameter which controls the flow similarity is Reynolds number. So you should make sure the wind tunnel RE number and actual flow RE no. should be close as much as possible.

For detail discussion please refer to the book: Wind Tunnel Testing by Alen Pope.

The values obtained from the wind tunnel test can be changed to dimensionless numbers, the main number in the wind tunnel is the Reynolds number that which has the values of body dimensions and velocity also the viscosity of the fluid or dynamic viscosity. from this data can be changed and calculated scales (diameter, height, aspect ratio,..... etc.)