The text font and size are dependent on the text style - which is basically a family type in Revit, or a family symbol in the API. So, for example, if you change it for one text note with the style "1/8" Arial", it will change for all text notes in the project with the same style.
I doubt if that is what you want, so the example I've posed does a couple of things. It places a text note with the default style, and then changes the font size for that style. (Again, that could change the size of a lot of different text notes in your project.)
But then it creates a new text style, changes the font of that style to arial narrow, and finally changes the style of the text note to the new style.
You will have to select what you want to use and delete the rest, but it should give you an example to follow to get exactly what you are after.
This is a macro, so if you are using visual studio, you will have to change the first two UIdocument and Document lines
public void ChangeSizeAndFont()
{
UIDocument uidoc = this.ActiveUIDocument;
Document doc = this.ActiveUIDocument.Document;
using (Transaction tran = new Transaction(doc, "change text font and size"))
{
tran.Start();
ElementId defaultTypeId = doc.GetDefaultElementTypeId(ElementTypeGroup.TextNoteType);
XYZ origin = new XYZ(0, 0, 0);
TextNote note = TextNote.Create(doc, doc.ActiveView.Id, origin, "Hello", defaultTypeId);
//change the text size of a text type:
TextElementType textType = note.Symbol;
BuiltInParameter paraIndex = BuiltInParameter.TEXT_SIZE;
Parameter textSize = textType.get_Parameter(paraIndex);
//units are in feet, so this is 1/4"
double newSize = ( 1.0 / 4.0 ) * ( 1.0 / 12.0 );
textSize.Set( newSize );
//create a new text type and change the font
// Get access to all the TextNote Elements
FilteredElementCollector collectorUsed = new FilteredElementCollector(doc);
ICollection<ElementId> textNotes = collectorUsed.OfClass(typeof(TextNote)).ToElementIds();
foreach (ElementId textNoteid in textNotes)
{
TextNote mytextNote = doc.GetElement(textNoteid) as TextNote;
// Create a duplicate type
Element ele = mytextNote.TextNoteType.Duplicate("ADSK_NewTextNoteType");
TextNoteType noteType = ele as TextNoteType;
if (null != noteType)
{
BuiltInParameter paraIndex2 = BuiltInParameter.TEXT_FONT;
Parameter noteFont = noteType.get_Parameter(paraIndex2);
//change its font
noteFont.Set("Arial Narrow");
}
//change the text notes type to the new type
//Dim elemId as DB.ElementId = noteType.Id
note.ChangeTypeId (noteType.Id);
//note.Symbol = noteType;
//only duplicate the first type
break;
}
tran.Commit();
} // end using
}
.