A pizza stone is a ceramic tablet that can mean all the difference between a soggy, inedible mess of biological waste and what Jack Lemmon called in one of his movies "a wet dream with a crust." In other words, it's not the size, but the technique that matters.
Then again, too little can easily leave you unsatisfied...but let's dispense with the bad innuendos and focus on technique. Pizzas reach the endpoint of their teleological existence through an infusion of heat, usually great in magnitude (450-500 degrees) and short in duration (12-14 minutes). Besides the energy needed to pre-heat and maintain an oven at such capacity, there are two other problems that must be overcome to ensure a high-quality pizza.
The first is the problem of uneven heat distribution. You wouldn't think that this would be that big of a thing for a round flatbread in the center of a 500 degree inferno. Yet, for some darned reason, conventional ovens have spots which get hotter a bit faster than others. Ever made the mistake of baking a pizza on a surface with low convection, like a cookie sheet? If so, you've seen the principle directly in action: heat currents from above bake the surface to a tender golden brown while the dough underneath stays mushy and has the feel and taste of wet cardboard when you bite into it.
The second, somewhat closely related to the first, is the problem of moisture. Improper baking can make a pizza dry out into a shriveled husk long before it's actually ready for consumption. This can lead to unsightly third degree burns, crispy sauce, tough cheese, and lots of foul smoke.
But don't let your Red Baron go down in flames. A pizza stone works to address these shortcomings of household appliances by acting as a poor man's brick oven, whose porous surfaces can absorb and homogenise tremendous temperatures. And rather than bleed away moisture, the bricks help to contain and distribute water contained in the bread placed atop them, ensuring a uniform, crunchy texture.
Most stones are relatively inexpensive as it is, but for lack of availability an unglazed ceramic tile will substitute quite nicely, as long as it has a corresponding thickness (around 1/4 to 1/2 in) and resistance to high temperature. It's also important to keep in mind that the stones need to be fired in a pre-heated oven for about thirty minutes prior to actual baking. Stones should never be washed with soap as their porous properties will cause them to retain it and convey the taste to the cuisine; instead, scrub and rinse them with a mix of baking soda and water and dry with a clean cloth. They will get 'tempered' by the fire and will turn a nice patchy, rugged brown with assorted scorch marks here and there -- just like a well-worn and beloved motorcycle jacket.
Despite the inconveniences of pre-heating and cleaning, the added results to even a mediocre frozen pizza are stunning. If you have any interest in making pizza on a regular basis, you'll end up getting more than your money's worth with one of these in no time.
Just don't drop it.