protected void Page_Load(object sender, EventArgs e)
{
    RadToolBarDropDown dropDown = new RadToolBarDropDown("PageSize");
       
    dropDown.CssClass = "perPageDropDown";
    dropDown.EnableImageSprite = false;
    // dropDown.Text = GetGlobalResourceObject("Insight", "Per pagina")
    // dropDown.ToolTip = GetGlobalResourceObject("Insight", "Per pagina")
    FileExplorer1.ToolBar.Items.Add(dropDown);
}

<telerik:RadFileExplorer RenderMode="Lightweight" runat="server" ID="FileExplorer1" Width="595px" Height="350px"
    AllowPaging="true" PageSize="10">
    <Configuration EnableAsyncUpload="true" ViewPaths="~/"
        UploadPaths="~/" DeletePaths="~/"></Configuration>
</telerik:RadFileExplorer>