That's kind of a tough one to answer. It depends on how big the yard is, how much wind there is, etc. All I can say is the bigger the better.
The problem with doing a yard is that there's nothing to keep the fog contained in the yard. Instead maybe think about where the fog is coming from and how that looks. You're not going to get an even haze that just stays there like you would indoors.
My neighbor and I both use the cheap 1000w fog machines you can get at Party city. We each have 2 of those, and some smaller 400w ones. The 2 1000w will do a good job, but like mentioned above, if you sneeze, all the fog will go away. But you will have good and bad moments through out the night. I am planning on extending my cemetary fence and have the bottom 3' out of plywood and painted like stone. Then add a 4' section of the PVC on top. Hopefully that way, it will block most of the wind on the ground and I can keep my fog a little longer.
I think NTX is right on with the 'wall' idea. Unless you have a perfect night with no wind, you are going to have some issues keeping your fog 'at bay'. I'm also looking for a good machine that will produce good fog with little warm up time between bursts. From what I have researched, you need at least a 1000w unit to provide good coverage, even on windy days. I've found that the Eliminator brand has gotten fairly good reviews.
Good one DeathTouch that and a real foggy night and you will not have to worry about it. I would get a machine with 1000 watts or more, that is what I have and on a windy day I do not use it it is a waste of time and electricity it all blow away to fast.