We fenced an area behind the garage just for this purpose. Rocket still has to ask to go out, but there is one room where a doggy door could be cut into the wall if we want.
The area is an L shape along the side of the house and back of the garage, and is aproximately 26 long x 12 wide. We just fenced in the other 2 sides with a 6 foot privacy fence and a locked gate, so there was no magic formula to come up with that measurement.
The only use of that area is a potty area, so he knows if he is out there, that's what he's out there for. We left the grass and some mulched areas that were already there in place, with plans to eventually buy some used astro turf to eliminate the mud and mowing. It's not muddy very often, but the ground has not frozen here this winter for more than a day or two, so it is muddier than usual when it rains or snows a little.
He has 3 large traffic cones to use as "targets." At least that's what we'd like him to use them for. They are actually for doing circular zoomies with an occasional Figure 8 thrown in.
The rest of our yard cannot be fenced easliy without blocking views of the pond, which is why we bought this lot. We do a walk or two each day, with the potty area being used for the remainder of the times that he needs out.