There aren't any LGDs that (in my opinion, of course) would do well in that small of an area.  We have a few acres that ours have access to and question if it's enough for them.  There are good chances they may feel the need to roam off of that size of property and many can scale a 6 ft fence.  One of mine (maremma mix) needs hotwire along the bottom of all of our fences or she will go under and go on an off-site adventure.  She was rehomed to me for this reason as she was doing the same at her previous home and they were unable to install hotwire.    
Another question I would ask is if you have neighbors?  We have taken in LGDs from people who didn't want to give up their wonderful dogs, but the neighbors forced that to happen because of the incessant barking.  In a suburban or urban setting, there are just so many things for them to bark at and many people underestimate how much they are capable of barking, but it's part of their job and what they need to be able to do.   Once they get to my place where the neighbors are some more distant, they bark much less and learn when it's appropriate without the distractions of suburban or urban life.  
Do you have or can you use a perimeter hotwire to keep the land predators out?  And maybe bird netting over a section of the yard for the birds?  
I am not saying it couldn't work, but it would be more challenging to fit them in a box they are not naturally suited to.