He had a picture of the parents and he said they get very large. Given that you have a large environment for them they can get up to 12". These fish were hand carried by my friend which he brought back from Singapore. Believe it or not....
Here is the E-mail I got from him reguarding the size with a picture...
I couldn’t find the picture of the parent discus I will give it to you when I find it, but found a similar one from the same breeder when I was in Singapore. This one is almost the same size as the 12” heater. The pair that you are getting from me get huge, can you imagine having a pair of discus in your house the size of a dinner plate? Your friends will drop their jaws…. , but a big tank is needed with clean water.