The number of vertices/triangles in the shadow mesh depends on the number of triangles in the source mesh, here are the numbers.
For a two-manifold/closed source mesh:
vertexCount = sourceTriangleCount * 3
triangleCount = sourceTriangleCount * 4
For a non two-manifold/closed source mesh:
vertexCount = sourceTriangleCount * 6
triangleCount = sourceTriangleCount * 6
As you can see, two-manifold source meshes are to be preferred. Two-manifold source meshes result in fewer vertices/triangles and are less fill-rate intensive.