var myDoc = app.activeDocument,
myPdfExportPreset = "[Smallest File Size]",
myPdfFile = File(myDoc.fullName.fsName.replace(/.indd$/i, "") + ".pdf");
app.scriptPreferences.measurementUnit = MeasurementUnits.PICAS
var docHeight = myDoc.documentPreferences.pageHeight
var docWidth = myDoc.documentPreferences.pageWidth
with ( myDoc.watermarkPreferences )
{
watermarkVisibility = true;
watermarkDoPrint = true;
watermarkDrawInBack = true;
watermarkText = "HAPPY NEW YEAR! (^/)";
watermarkFontFamily = "Arial";
watermarkFontStyle = "Bold";
watermarkFontPointSize = Math.round(docWidth/2);
watermarkFontColor = [180,180,180];
watermarkOpacity = 60;
watermarkRotation = -45;
watermarkHorizontalPosition = WatermarkHorizontalPositionEnum.watermarkHCenter;
watermarkHorizontalOffset = 0;
watermarkVerticalPosition = WatermarkVerticalPositionEnum.watermarkVCenter;
watermarkVerticalOffset = 0;
}
myDoc.asynchronousExportFile(ExportFormat.PDF_TYPE, myPdfFile, false, myPdfExportPreset);
myDoc.watermarkPreferences.watermarkVisibility = false;
Line 6 and 7 gives the document height and width in PICAS and line 16 sets the fontsize of the watermark. I have set the size to 50% of docWidth you can change the calculation with what you need.