Did the pinion service tonight. It rolls so much better now!
Both front and back are approximately 8.5", but the rears have the gear bonded into the center that interfaces with the pinion gear. So that makes them a pretty unique product.
I'm still on my originals after 8 years of pretty hard use. There are other things wearing out, but the tires/wheels are still hanging in there. Unless they've cheapened them considerably, I wouldn't get too stressed over the salesman's comments.