show large selected item names textscaption dropdowncheckbox

In my web application, i have many dropdowncheckboxes but i need to display what i select all the items i need to show on the dropdowncheckboxes of the texts caption box.when i select small items names it shows perfectly.It will not show properly by selecting the large item names on texts caption box. i want to show large item names properly. when i select that large items and select by mouse those names and move arrows buttons (keyboard keys) on texts caption of dropdowncheckboxes it will show all the names without change the dropdowncheckboxes width is it possible to do.

aspx:

<%@ Register Namespace="Saplin.Controls" Assembly="DropDownCheckBoxes" TagPrefix="asp"%>
<asp:DropDownCheckBoxes ID="dropdown1" runat="server" UseSelectAllNode="true"
UseButtons="true" OnSelectedIndexChanged="dropdown1_SelectedIndexChanged" AutoPostBack="true">
<Style DropDownBoxBoxWidth="200"/> </asp:DropDownCheckBoxes>

.CS:

protected void dropdown1_SelectedIndexChanged(object sender, EventArgs e)
{
List<String> checkedList = new List<string>();
foreach (ListItem item in dropdown1.Items)
{
if (item.Selected)
{
checkedList.Add(item.Text);
}
}
dropdown1.Texts.SelectBoxCaption = String.Join(",", checkedList);
}

how can i do this can anyone please help me
Thank you