I have a 3 x 6 inside area for night time use for 3. Then they have a large outside pen with a pool for they daytime. I would think a 4 x 8 would be sufficient for you as long as they have somewhere for them to go outside during the day. Otherwise it should be bigger if they are going to be in it all the time. The more room the better, I always say.
If you want to keep the house dry and them healthy...keep their house and the pool a decent distance apart. If not the bedding will stay perpetually wet which as you know can lead to illness. Even though they are waterfowl they need a clean dry place to nest and sleep.