The availability of brass and stone determines the maximum size of my sundials. Good stone of this large size is very difficult to find. I buy it when I find it. I'm very discriminating when selecting my stone. I will not use stone that has any visible fractures, pits or waves.
Thin flagstone breaks very easily, but thick stone is surprisingly strong.
I buy my flagstone from the quarry's local distributor who sells it to landscapers. For the sundial support structure, most people use birdbath or fountain pedestals made of stone or tinted concrete found at the local garden or fountain center. Outside, the sundials immediately begin to age. The stone and painting will slowly bleach out and the brass, if left unpolished, will develop a beautiful dark moss-green patina which will actually protect the brass from further corrosion. Here in the desert, this takes about three years.