I would have to say... it depends on your grain bill for what you're brewing.
That said, using 1.25 quarts per pound of grain (about normal for mash thickness, IIRC), you could mash up to 12 pounds of grain in a 5-gallon mash tun (if you use Denny's Cheap and Easy mash tun build, if you put a false bottom in it, it changes things). At that rate, you'll take up about 4.71 gallons of space.
Now, another thing that could be done would be to mash what you can and do a late add of malt extract (add it in the last 15 minutes of the boil). By doing a partial mash like that, you could add the top-off water and make a full 5-gallon batch, reduce your reliance on extract, and produce good beer.
So in short, you would have options. Doing small batches, you'll probably be able to mash what you'll need, but you can also do full batches if you desire.