The vast majority-- probably over 90%-- of all Alchemy Stones are between 1" (25mm) and maybe 3" (75mm) long (or in diameter). I have always used "fits well in the hand" as an important criterion for choosing a stone. On the occasions when I bring a large rock home for the beach it tends to be because it has a cool "natural" design or shape, not because I want to paint on it.
Part of the reason larger stones don't often become Alchemy Stones is that they are very difficult to find with completely smooth surfaces. They are also often chipped and scuffed, as a result of impacts with other stones.
The stone pictured here is a little over 6" (15cm) across and weighs several pounds.
Part of what makes it very difficult to paint such a stone is that it has over a thousand individual design elements. And if I make one little mistake somewhere, hours of work goes down the drain. People do occasionally ask for a large stone... often because they want a "serious" piece of art. I am not quite sure what that "means," in a functional sense, but I do my best to meet requests like that.
For those who are curious-- a stone like the one pictured, with one of Sarah's specially made "Treasure Bags"-- goes for about $150.00-$200.00. I am not sure how many hours of work goes into one, because I tend to work a little bit on them, every day, for several weeks. And just because I "finish" painting does not mean the stone has succeeded... the design can also mess up and "feather" or "float" while the finish coat is being applied.
How many do I make? Maybe 3-4 a year.