Change ASP.Net Label Font Size using JavaScript on DropDownList change

I am trying to change font size of label controls, but what I tried is not working.

HTML

<asp:Label ID="LabelName" runat="server" Text="Name"></asp:Label>
<asp:Label ID="LabelDate" runat="server" Text="Date"></asp:Label>
<div class="input-group">
   <asp:DropDownList ID="FontSize" runat="server" CssClass="form-control" onchange="ChangeSize(this.value)"></asp:DropDownList>
</div>

JavaScript

<script type="text/javascript">
    function ChangeSize(FontValue) {
        document.getElementById("LabelName").style.FontSize = ValueFont;
        document.getElementById("Labeldate").style.FontSize = ValueFont;
    }
</script>

C#

The DropDownList control for font size is populated from Database

private void FetchFontSize()
{
     using (SqlConnection con = new SqlConnection())
     {
        con.ConnectionString = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand("SELECT fontsize FROM FontTable", con))
        {
            cmd.CommandType = CommandType.Text;
            cmd.Connection = con;
            con.Open();
            FontSize.DataSource = cmd.ExecuteReader();
            FontSize.DataTextField = "fontsize";
            FontSize.DataValueField = "fontsize";
            FontSize.DataBind();
            con.Close();
            FontSize.Items.Insert(0, new ListItem("Size", " "));
            FontSize.Items[0].Selected = true;
            FontSize.Items[0].Attributes["disabled"] = "disabled";
        }
    }
}