int fileSize = FileUpload1.PostedFile.ContentLength;
            if (fileSize > (maxFileSize * 1024))
            {
                PanelError.Visible = true;
                lblError.Text = "Filesize of image is too large. Maximum file size permitted is " + maxFileSize + "KB";
                return;
            }

Select Your File <asp:FileUpload ID="FileUpload1" runat="server" /><br />
    <asp:CustomValidator ID="CustomValidator1" runat="server"             
            ErrorMessage="Please select valid .jpg or .jpeg file and less than 1MB" 
            onservervalidate="CustomValidator1_ServerValidate"></asp:CustomValidator><br />
    <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />

 protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
{
        string filename="";
        if (FileUpload1.HasFile)
        {
            filename = FileUpload1.PostedFile.FileName;
        }
        else
        {
            args.IsValid = false;
        }

        if (filename == "")
        {
            // check file is empty or not
            args.IsValid = false;
        }
        //Check file less then 1MB multipy for 50*1024 Mb
        if ((FileUpload1.PostedFile.ContentLength / 1024) > 1024)
        {
            args.IsValid = false;
        }   
}