Hi akhter,
Set the Font-Size property for options in CSS.
Please refer below sample.
HTML
<style type="text/css">
select option {
font-size: 20px;
cursor: pointer;
}
</style>
<asp:DropDownList ID="ddlCustomers" runat="server">
<asp:ListItem Value="0" Text="--Please Select--"></asp:ListItem>
<asp:ListItem Value="1" Text="ASPSnippets"></asp:ListItem>
<asp:ListItem Value="2" Text="ASPForum"></asp:ListItem>
<asp:ListItem Value="3" Text="Excelasoft Solution"></asp:ListItem>
</asp:DropDownList>
Screenshot