A good min tank size is 48" x 15" x 18", bigger is better of course, make sure you have 18" height for adults. You'll need a group of 5-6 discus, and make sure the tank is fully cycled. Discus need water on the soft side, you will most likely need to soften your tap water, an RO filter is the best way to achieve this. You will need to find out the water parameters of your discus supplier and match those. Buy from a discus specialist and not a normal LFS unless you are sure of the quality of the fish. Make sure your heater can maintain the high temps required.
A general rule is to provide 10 gal per discus, but this does not mean you can keep one in a 10. With experience you can stock a little heavier than this.
Buying fish around 4" is the best IMHO, they settle better and are not as easy to stunt as babies. I also think a bare bottom tank is best for any discus, but is required for raising babies. You could keep babies in a smaller tank, but they are easily stunted and bigger is better.
Please do a search on this forum for discus, there are plenty of good posts on them and you can find out everything you need, and please please take your time to decide if discus are right for you as they require a bit more dedication than your average fish.