Choosing a pizza stone can be tricky and there are a lot of things to consider when purchasing a pizza stone. The type of oven you have, the temperature of your oven, and what size stone fits in your oven will make the selection process pretty simple.
Soapstone is really conductive to heat. However, in order for it to pre-heat you will need an oven that gets higher than 525ºF, any lower and the stone will not operate correctly. This is a great stone for commercial users and it can cook a pizza in about four minutes. That bake time is only at 525º too, if you have a good commercial oven you can turn pizzas in a matter of minutes. Soapstone is a terrific, durable stone that will bring out the best in your ingredients.
Ovens that operate lower than 500ºF will be better off with a Cordierite stone for their pizzas. This type of stone will get your oven baking crisp pies at lower temperatures. Cordierite is also widely available and relatively cheaper than soapstone and again can work in lower temperatures. Cordierite is available at most kitchen stores and can even be cut down to the dimensions you want for your oven.
Soapstone and Cordierite do not absorb water so this is not really a huge issue when making your selection. However, thickness is something you want to consider when purchasing a pizza stone. A thicker stone will retain a lot of heat, but, does take awhile to heat up using a lot more energy. The prime thickness for a pizza stone is 1/2 inch, it will retain the heat and does not consume as much energy while being pre-heated.
There is really no right answer for what size is best, however, having a pizza stone that is too small for a pie you want to bake is always a major bummer. I recommend getting a larger stone even if you don't need the entire surface area each time. An 18 inch diameter is a wonderful size to have, if it works in your oven. Ovens that can hold an 18 inch pizza stone are often hard to find so make sure you know what size will work best for your oven before investing any money into a stone.
