There are many hundreds of rhododendrons available for Z6 and some I've had a good deal of success with that are rated as Z7. I am, however, at a loss to understand what you mean by "age/size of the same rhododendron in terms of surviving". In general, rhododendrons achieve their maximum hardiness after 3 to 5 years in the ground. This time period is necessary to produce a large enough root mass in a natural form as opposed to the shape of root balls even in a large container. It's also important to think in terms of adaptability and overall health rather than just survival. Flower buds are usually less hardy than the plant itself, so that is a further consideration. Blooming and growing seasons are other ones. Late frosts - common in much of the East - can be very damaging and destroy that season's flower display.

I can buy a 12" or 24" of the same rhododendron.

There might be some extremely minor edge in survivability for the larger size, but other factors are far more important.