IMO you should start with a 55 gallon and no less. A 40 breeder works alright for a breeding pair, or to raise out small fish, but it's not a good size for long-term discus keeping.
It's always best to start with groups of discus or you'll run into aggression issues. Discus form a pecking order and if you have less than 4 fish you're most likely going to have problems. I always recommend starting with 6.