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Once you have your plan, your supplies and safety gear it is time to pick a stone to use for sculpting. For beginning sculptures it is best to start with a smaller sized stones about 20lbs are less; however, if you are more experienced the size should reflect your artistic plan. Another variable to consider is the shape of the stone, traditionally square pieces are used; however, there are many people out there who break with tradition and use oblong or even circular pieces. When picking a shape think about the finished product you wish to create.I found some more information here.

The softest and easiest types of stone to sculpt is soapstone, alabaster and wonder-stone (Pyrophyllite). Out of these choices, the softest and easiest to sculpt would be a soapstone and is typically a choice that beginning sculptures should experiment with and consider using for initial sculptures. For more advanced sculptures harder stone is typically used, because it requires more skill with sculpting tools. Harder sculpting stones include, marble, granite and jade. Harder stones also seem to stand up to the test of time and the elements better then softer stones, so before picking a stone consider where the piece will be on display.