There is no set size a discus can be when they are able to spawn. I have seen stunted discus spawn at 3.5-4" in size....at the same time, I have seen beautiful full grown discus at 8" spawning for the first time. Most discus are able to spawn around 12-14 months of age....while others not until they are 18-24 months old...it really depends on the color type you are trying to breed.
The key with breeding discus is consistency in daily activities like feeding and water changes. This along with a low water hardness of no more than 100 microseimens (or about 3 GH) is needed. Discus dont need soft water to live and thrive, but when you are breeding them, the eggs need the soft water to hatch. Keep the temp of a breeding tank in the low 80's (82ish) and the light subdued as much as possible. Minimize traffic around the tank and keep the tank as clean as possible at all times to ensure the health of the pair and the eggs when they are laid.