@sdtechsupport.sirius,
I suggest you to please try using following sample code and make necessary changes in your application in accordance to it. Please also set text autofit type before populating text in text frame as well.
public static void TestAutofit()
{
using (Presentation pres = new Presentation())
{
// Access the presentation slide
ISlide slide = pres.Slides[0];
// Add Smart Art Shape
IAutoShape ashape = slide.Shapes.AddAutoShape(ShapeType.Rectangle, 0, 0, 100, 100);
ashape.TextFrame.TextFrameFormat.AutofitType = TextAutofitType.Normal;
ashape.AddTextFrame("");
ashape.TextFrame.Text = "THis is long long text, THis is long long text,THis is long long text,THis is long long text,THis is long long text,THis is long long text,THis is long long text,THis is long long text,THis is long long text,THis is long long text,THis is long long text,";
pres.Save("C:\\Aspose Data\\SimpleSmartArt_out.pptx", Aspose.Slides.Export.SaveFormat.Pptx);
}
}