How to change WaterMark font size using jQuery in ASP.Net

hi I use watermark for textbox in page and I define fontsize for watermark

below are code:

       <script type="text/javascript">
        window.onload = function SetColorOnPageLoad() {
            var defalutAzText = "از";
            var defalutTaText = "تا";
            var defalutsearch = "جستجو بر اساس کد ملک";
            functionSetColor(document.getElementById("Txtsearch"), defalutsearch);
            functionSetColor(document.getElementById("Txtejare1"), defalutAzText);
            functionSetColor(document.getElementById("Txtejare2"), defalutTaText);
            functionSetColor(document.getElementById("txtvadi1"), defalutAzText);
            functionSetColor(document.getElementById("txtvadi2"), defalutTaText);
        }

         function functionSetColor(Textbox, WaterMark) {
        if (Textbox.value.length > 0 && Textbox.value == WaterMark) {
            Textbox.style.color = "#443f13";
            txt.style.fontSize = "14.5px";
 
        } else {
            Textbox.style.color = "white";
            txt.style.fontSize = "14.5px";
        }
    }
 
    function WaterMark(txt, evt, msg) {
        if (txt.value.length == 0 && evt.type == "blur") {
            debugger;
            txt.style.color = "#443f13";
            txt.value = msg;
            txt.style.fontSize = "14.5px";
            txt.style.fontfamily = "behtop_Yekan";

        }
 
        if (txt.value == msg && evt.type == "focus") {
            txt.style.color = "white";
            txt.value = "";
            txt.style.fontSize = "14.5px";
            txt.style.fontfamily = "behtop_Yekan";
        }
    }
</script>

problem is that here at first when run page it shows watermark with size that I define for textbox (in css) and when I click on textbox and after that  click out of text box it will display watermark's fontsize  that I define an above code

please refer below video:

https://vimeo.com//220612125

best regards

neda