How to change Designator text size by script in Altium 17 or newer
See this page for an example:
Altium Designer example script

The relevant part is:

OldUseTTFonts := Designator.UseTTFonts;
OldItalic     := Designator.Italic;
OldBold       := Designator.Bold;
OldInverted   := Designator.Inverted;
OldFontName   := Designator.FontName;
OldFontID     := Designator.FontID;
OldRotation   := Designator.Rotation;


to save the current status, the other way round to assign some new settings. You'll neeed to find out if FontId is still relevant (trial and error)

Here is a reference to ADs IPCB_Text object with the associated methods/properties. You'll need to scroll down pretty far or do a search for "IPCB_Text interface".