Half of the fun of backyard baseball is the convenience. No special equipment or playing fields necessary; just a ball, a bat and a backyard. But not every backyard is built for baseball. For a safe and fun game, you need lots of flat space and few obstacles, particularly if that obstacle is your neighbor's new BMW.
The ideal location for backyard baseball is a large, open, grassy area free of trees, playground equipment, pools or houses. A standard Little League T-ball field has 50 feet (15.2 meters) between bases and a distance of 200 feet (61 meters) from home plate to the outfield fence [source: Little League]. It's OK if there are houses nearby, but you want at least one direction to be free of breakable windows. That's the direction you want the batters to face when hitting.
If you are using real baseballs and bats, the only safe choice is a large. open backyard with no breakable objects, even in foul ball territory. If you are using Wiffle balls or Nerf balls, then you can get away with playing in a smaller yard. But still be aware of all obstacles in the lawn. It's possible to concentrate so hard on catching a fly ball that you run straight into a stop sign. Just ask my wife.