[C#]
float objectRadius = 0.0f;
Vector3 objectCenter = new Vector3();
using (VertexBuffer vb = Mesh.VertexBuffer)
{
GraphicsStream vertexData = vb.Lock(0, 0, LockFlags.None);
objectRadius = Geometry.ComputeBoundingSphere(vertexData,
mesh.NumberVertices,
mesh.VertexFormat,
out objectCenter);
vb.Unlock();
}