I say they can be very small.
|Heard through the grapevine|
I have one at home I use as a home theater video player, and another one at work I use for web design and batch conversion of digital images.
If you need a high-power expandable Mac, buy a Mac Pro. If you need a less powerful Mac with a screen, buy an iMac. If you need a portable, buy a Macbook. But if you only need a modestly powerful Mac, it doesn't need to be portable, and you already have a display, a Mac Mini is a great thing.