Most materia seems to be approximately apple-sized (you can see several pieces of it in Final Fantasy VII - Advent Children.) Two examples can be found here and here on the Final Fantasy wiki. They are, respectively, Kadaj holding a piece of Materia and several pieces of Materia in a box.
However, there are some exceptions; the Black Materia is a sphere with a somewhat larger radius than a CD (it's seen in Dissidia) for example. The other plot-important pieces of materia, as well, are different in size (and sometimes shape.)