var layer = app.activeDocument.activeLayer;
var textSize = getFontSize(layer);
 
function getFontSize(layer){
    var ref = new ActionReference();  
    ref.putEnumerated( charIDToTypeID("Lyr "), charIDToTypeID("Ordn"), charIDToTypeID("Trgt") );   
    var desc = executeActionGet(ref).getObjectValue(stringIDToTypeID('textKey'));  
    var textSize =  desc.getList(stringIDToTypeID('textStyleRange')).getObjectValue(0).getObjectValue(charIDToTypeID( "TxtS" )).getDouble(stringIDToTypeID('size'));  
    if (desc.hasKey(charIDToTypeID("Trnf"))) {  
        var mFactor = desc.getObjectValue(charIDToTypeID("Trnf")).getUnitDoubleValue (stringIDToTypeID("yy") );
        textSize = (textSize* mFactor).toFixed(2);  
    }  
    return textSize;
};
 
alert(textSize);