Some companies have begun manufacturing "grass boxes" for dog owners that live in apartments or condos. Many are made for smaller dogs or cats, but you can make your own at home to accommodate a larger dog.
One option for making a dog's grass box is to use a long, flat storage container for the box with a layer of turf for the grass. The advantage of using a large, flat plastic storage container for your dog's grass box is that the box is already pre-made for you, and is easy to clean using water and bleach. These boxes are typically three to four feet in length, and can accommodate a larger dog.
Sprinkle baking soda in the bottom of the box first to help absorb odor, and put a layer of either paper or a "pet pad," an absorbent pad used for potty training puppies, beneath a piece of turf. You can find the turf at a garden, hardware, or craft store. You will need to clean the turf and box once a week or more, and may want to consider locating it outside, or in a place that is well ventilated. The advantage of using turf is that the plastic can be washed easily by spraying it with water and bleach.
If you know a little carpentry and have the tools, you can build your own wooden box for your dog's grass bed. If you have enough space where you plan to put the box, you can make a box that is two feet by four feet. You may want to put a liner down in the box to catch any excess urine that runs through the sod. You can line the box with plastic, such as a thick plastic dropcloth, for easy clean up. Another option is to line the box with a layer of sand or litter.
You can line either a plastic or wooden grass box with pieces of sod. This natural material requires water and sunlight or else it will die, but has the advantage of being a renewable resource. You can find sod at a garden store early in the morning, or search your city's phone book to see if there is a sod delivery service in your area. A living turf like sod will also require weekly replacement.