I have always worked on the theory, and please bear in mind it is only my theory. A dog, any dog in a big yard wont look around the area and think to itself "Oh i have a big yard i best go and exercise"
It sounds like from your post that the dogs do get out and get regular exercise so I would not be too concerned about the yard size, (as long as there is at least a little room to move around and cater for the occasional romp and tumble)
I have an Akita and we have a very small back yard (not much larger than a courtyard really) But there is enough space for him to be able to do his business then not have to lie in it either.
ETA even a quater acre would not be too small, that is quite a large area when you think about it (less work for you too if you are helping with the fence
