According to Dennis Voigt the ideal pond is 10-15 yds wide and 30-40 yds long. Dennis has a free downloadable Swimby article on his Retrievers Online website in which he gives these dimensions.
If I was building a pond, I would use the above dimensions. I lost access to my best swimby pond which was close to the above dimensions. I have had to make do with larger ponds. Like 30 yds wide and 50 yds long. It just means fewer repetitions in a session, IMHO.