depending upon your need increase / decrease the value in style
<style type="text/css">
.btns
{
width:60%;
}
</style>
<asp:Button ID="Button_Config_iPhone" runat="server" onclick="Button_CreateConfiguration_iPhone4" Text="iPhone" ToolTip="iPhone " CssClass="btns"
/><br /><br />
<asp:Button ID="Button_Config_WinMob" runat="server" onclick="Button_CreateConfiguration_WindowsMobile" Text="Windows Mobile Phone" ToolTip="Windows Mobile Phone" CssClass="btns"
/>