@SteveRichter: Actually there is a minimum size. The UPS switching and conditioning hardware is roughly the same size as the battery. If you take a look at even a small size UPS the batter is only aabout 1/3 or the total volume.
The extra effort of having a UPS goes far beyond just causing your computer to shut down when the power drops. There are all sorts of transient events that can not only cause reboots, but also damage to components. IMO it's just not worth the risk.