GetModelBoundingSphere.cs

public static BoundingSphere GetModelBoundingSphere(Model model)
{
    // Initialize minimum and maximum corners of the bounding box to max and min values
    Vector3 min = new Vector3(float.MaxValue, float.MaxValue, float.MaxValue);
    Vector3 max = new Vector3(float.MinValue, float.MinValue, float.MinValue);

    // For each mesh of the model
    foreach (ModelMesh mesh in model.Meshes)
    {
        foreach (ModelMeshPart meshPart in mesh.MeshParts)
        {
            // Vertex buffer parameters
            int vertexStride = meshPart.VertexBuffer.VertexDeclaration.VertexStride;
            int vertexBufferSize = meshPart.NumVertices * vertexStride;

            // Get vertex data as float
            float[] vertexData = new float[vertexBufferSize / sizeof(float)];
            meshPart.VertexBuffer.GetData<float>(vertexData);

            // Iterate through vertices (possibly) growing bounding box, all calculations are done in world space
            for (int i = 0; i < vertexBufferSize / sizeof(float); i += vertexStride / sizeof(float))
            {
                Vector3 vert = new Vector3(vertexData[i], vertexData[i + 1], vertexData[i + 2]);

                min = Vector3.Min(min, vert);
                max = Vector3.Max(max, vert);
            }
        }
    }
    
    // The Center will be the average of the Max and Min
    Vector3 Center = Vector3.Add(max, min)/2;

    // The Radius will be half the difference between the max and min
    float Raduis = Vector3.Subtract(max, min).Length() / 2;


    // Create and return bounding box
    return new BoundingSphere(Center, Raduis);
}