20x40m is great for jumping and dressage!
You can use a standard arena that are 20x40 meters. Jump and use some of the fences more than one time. In that way you are going to save some space! Be creative, good luck!
20x40m is standard dressage arena, I have jumped courses in a ring this size but they couldn't be too technical due to the size, you can make it bigger and then when you want dressage mark out 20x40 or 20x60 depending on what level you are at :)