The TextNoteType class is where a property to find the size of a Text type would ideally be found. But it is not.
However, all hope is not lost.
It is important to remember (and sometimes easy to forget), that there is also a huge set of parameters in the Revit API known as Built In Parameters. Many of these parameters work in the internal Revit code and not in the API, but many (such as TEXT_SIZE) are also perfectly usable in the API.
TextNote textNote = doc.GetElement(uidoc.Selection.PickObject(ObjectType.Element).ElementId) as TextNote; TextNoteType textNoteType = doc.GetElement(textNote.GetTypeId()) as TextNoteType; Parameter textSizeParam = textNoteType.get_Parameter(BuiltInParameter.TEXT_SIZE); TaskDialog.Show("Text Size", textSizeParam.AsDouble().ToString());