Couple of possibilities. First, find a tiling grass texture (try 3dcafe.com or look in freestuff here for something called "47 new textures" -- the guy has tons of amazing stuff), then haul it into Photoshop and tile it till you have enough to cover the space you need. Then, you can either apply it to a 2d square prop or, as I would prefer, an imported terrain made in Bryce or some other program with fractal-export capabilities. Extend the import to the size you want, then take it down on the y axis so all you have is just a nicely uneven ground. Apply your texture, and voila.