From what I understand a mature pair needs at least a 4'x6' pen at night that is secure against predators. We have a relatively big barn with two 12'x12' matted horse stalls on the north side. Since we don't have horses at the moment the geese go into one of the stalls at night. I keep it bedded deeply in straw and just pick out the manure daily and move the water to different spot in the stall every other day. Interestingly, they like to bed down around wherever the waterer is. So the manure is always concentrated around the water source. Simply moving their water moves where the majority of the manure lands.
We are going to move ours into a different pasture tomorrow morning. The grass in their current pasture is getting a little short and we won't have much rain again until the fall. The downside to a Mediterranean climate is not having any rain when it's most needed.