Problem is discus are a shoaling species and therefore its unfair really to keep one singularly. IMO a 55G tank is the minimum I would consider keeping them in
The bigger the better really, 55 Gallons is a good size but discus will never reach their full size unless they have constant water changes and a bare bottom tank of that size, if you want planted you must go for a much larger tank and ensure that they are very well fed and have excellent water conditions.