You can go quite large with a D3s or a lower resolution cameras, but in addition to good post-processing skills, a lot depends on the image itself and how it will be displayed. Something that doesn't rely on small details for impact (think sports photography or a portrait) can go very large without a problem. If your print won't be viewed up close, you can similarly go larger (think billboard as an extreme case). If your situation is the opposite of those, you'll need to be more conservative in how large you print. There's really no absolute answer to how large you can go, but I wouldn't hesitate printing at 24x36 inches for a sports shot that was viewed in a gallery situation.