To replicate a wind tunnel test, you are obviously limited to the physical size of the wind tunnel lateral to the car. It is suggested to have at least two body-lengths upstream of the car for the inflow boundary and three body-lengths downstream of the car for the outflow boundary. As always, more is better, but it is not always cost-effective.
To replicate the vehicle on the open road, you can use the old rule-of-thumb from the wind tunnel testing of using a solid blockage of .5%. For example, the projected area of your car should be .5% of the total area of the inflow plane.