We have two glorious options for you: 2 inches square or a slightly smaller, 1.75 inches.
We've held royal Lapis lazuli obelisks, pyramids, spheres, slabs, etc in our hands. Nothing, and we mean nothing, compares to the feeling of a square cube of Lapis.
There's something about the geometric pattern of the square with sharp edges, made out of quality Lapis. It has a life of its own. Dare we say, like a mini-generator. Awesome!
There are three stones that may carry a vibration in the earth for your sustenance. Notice it says "may carry," the implication is that it might be helpful to charge gems with Light. It is up to you how much you want to identify with higher vibrations, but Cosmic Law dictates that within the amethyst, within the lapis and within the jade you will find a capacity to store, even as it were, a microchip, to store light, vibration and the Word. Natural crystals from the earth are also the option for the altar. It is not necessary that it be something beyond your means, but even a little token shall remain with you as the presence of these emanations.