The key here is to have all the relevant dimensionless numbers (e.g. Reynolds number, perhaps Mach number, Froude number, etc.) as well as the geometry (which i

The key here is to have all the relevant dimensionless numbers (e.g. Reynolds number, perhaps Mach number, Froude number, etc.) as well as the geometry (which is also dimensionless: it's a set of angles and ratios), of your wind tunnel model consistent with the real-world situation you're testing. Then you measure other dimensionless quantities (e.g. drag coefficient). The tests give a correlation from one dimensionless quantity to another. That can mean, counter-intuitively, that a wind tunnel model needs to have a higher wind speed than the full-size object - so that the Reynolds number is correct.