Typically in 10 years, they grow to be 2 feet tall. In shade they grow taller, maybe even to 4 foot tall in 10 years. In full sun they will be on the low side. As they get older they keep growing. They don't stop at 10 years of age.

'Hinodegiri' is a Kurume azalea. We have a number of Kurume cultivars that have been in our yard since the early 1970's. These azaleas are about six feet tall, maybe a little taller.

Thanks rhodyman and jay for your replies. I had another nursery manager tell me he they'd get about knee high, so are there perhaps different sizes of hinodegiri or something? Or maybe it just depends on their age and location? These ones have very small leaves, and the nurseryman said the leaf size correlates with the size of the plant in general. If I space them closer, they won't get as big as if I spaced them farther apart, right? Do you know about how far apart they should be spaced?

I've had similar 'Hino-crimson' for decades, not up to the knees yet.