Radius doesn't count for anything. You want your axles all to rotate at the same rate to keep stress off the viscous coupling. Each revolution of the tire goes down the road a distance equal to the rolling circumference, not related to the radius which is distorted by the weight of the car pressing down on it (only at the bottom though ) Measure the circumference of the tire off of the vehicle with a tape or ribbon.
While unloaded radius is exactly related to the circumference, you will get more measurement error trying to measure the radius and then calculating the circumference, or matching radius tire to tire.