with below small css trick hope u can do that
<style type="text/css">
.css-lb {
width:100px;
overflow:hidden;
color:gray;
}
.css-lb select {
border:none;
height:500px;
}
.css-lb option {
width:100px;
}
</style>
<div class="css-lb">
<asp:listbox id="ListBox1" runat="server">
<asp:listitem text="text1" />
<asp:listitem text="text2" />
<asp:listitem text="text1" />
<asp:listitem text="text2" />
<asp:listitem text="text1" />
<asp:listitem text="text2" />
<asp:listitem text="text1" />
<asp:listitem text="text2" />
<asp:listitem text="text1" />
<asp:listitem text="text2" />
</asp:listbox>
</div>
worked for me hope it will help u out