[tools] terrain grid radius is 2.68

was `size` in 2.67a

http://www.blender.org/documentation/blender_python_api_2_67a_release/bpy.ops.mesh.html#bpy.ops.mesh.primitive_grid_add

http://www.blender.org/documentation/blender_python_api_2_68_release/bpy.ops.mesh.html#bpy.ops.mesh.primitive_grid_add