Hope someone will be on soon to correct me or agree with me, but I go by the rule of 2.5 square feet per duck for indoor quarters. Meaning night time, inclement weather. For outdoor confinement, I go by 7 square feet per duck. That situation would be if I go out of town or have lots of activity in the yard, or if I know there are predators in the area.
Mud is inevitable and unavoidable. My opinion on the mud is to simply choose a spot where the mud won't bother you so much. You can rotate your pools and you can create systems for recycling the used water that are specific for your situation. For example, you can water a tree.you can also have sand and or pea gravel brought in to put over landscape fabric to prevent unhealthy mud bogs.