This is one of the most important factors to consider. First of all you need to think of the practicalities – measure your oven and ensure that the pizza stone or baking stone you are buying will actually fit inside, with enough room to spare to allow it to be maneuvered. Remember, your pizza stone will be very hot and quite heavy when in use so you want to make it as user friendly as possible. If you plan on using it on the grill, do the same thing.
Larger pizza stones are generally the best as they give you more flexibility – you can cook everything from small individual pizza’s and breads up to large family size pizzas. It is also easier to transfer your uncooked pizza from your pizza peel or cookie sheet to the pizza stone if it is big – with small pizza stones you need to be more accurate and it doing this can be hard enough as it is. Similarly, a square or rectangle pizza stone gives a bit more room than a circular stone. This is especially important if you plan to use it as a baking stone for breads which are generally rectangular, verses traditionally round pizzas.
And of course bigger will cost more, though pizza stones are not very expensive and really, the best answer to this question is to have more than one.