At 6 months he may have reached full height, or he may grow upwards a bit more in the next couple of months, but he'll still have lots of maturing and filling out to do, which could take another year or more (like Mark said, how long is a piece of string)
The Breed Standard states that dogs should be between 15.5 and 16 inches at the shoulder. Many show types end up within that range, but there's more variation in workers because they've been bred for generations for their working ability rather than appearance or size and that's led to workers ranging from pocket rockets to pretty much Springer Spaniel size.