I have observed the requirement shared by you and suggest you to please try using following sample code on your end to serve the purpose. I hope this will be helpful.
label.TextFormat.TextBlockFormat.AutofitType = TextAutofitType.Shape;
Autofittype is not available for version 14.8. Do you know which version has this property for the lables.
I like to share that requested support has been included in Aspose.Slides 15.5.0. However, I suggest you to please try using latest Aspose.Slides 16.3.0 on your end. Please visit this documentation link for your further kind reference as well.