It's in preferences but I recommend leaving it at default as Maya likes to work with smaller objects and isn't really geared towards working to scale. When starting a project just determine your own system using the grid. For example, say you wanted to create a human who's 5' tall...you could visually use each grid division to equal 1ft. Or say you were creating a mammoth battle cruiser, you could use each division to equal 25m or whatever. You don't have to set it anywhere. As long as you establish for your project how you're visualizing the grid elements and keep it consistent when building other scene elements, you should be good. You can always scale everything as need be later if you need to but I would stick with this method as it will prevent your machine from getting bogged down...trust me. I once built a wine dispenser in maya to scale and every little tweak caused massive slow down.
Maybe this will explain it better:
"Terminat Bora Diem, Terminal Auctor opus."