For the time being you can manually increase the width of the decorated select:
ASPX:
		<script>
			function pageLoad() {
				increaseWidth($telerik.$('.rfdSelect'), 68);
			}
			function increaseWidth(element, increase) {
				element.css('width', parseInt(element.css('width').replace('px', '')) + increase + 'px');
			}
		</script>
<telerik:RadFormDecorator ID="decor" runat="server" DecoratedControls="All" Skin="BlackMetroTouch"  />
<asp:DropDownList ID="ddlASPNET" runat="server"></asp:DropDownList>
C#:
		protected void Page_Load(object sender, EventArgs e)
		{
			var dt = Getdata();

			ddlASPNET.DataSource = dt;
			ddlASPNET.DataTextField = "Name";
			ddlASPNET.DataValueField = "ID";
			ddlASPNET.DataBind();
		}

		private DataTable Getdata()
		{
			DataTable dt = new DataTable();
			dt.Columns.Add("ID");
			dt.Columns.Add("Name");

			for (int i = 1; i <= 100; i++)
			{
				DataRow dr = dt.NewRow();
				dr["ID"] = i.ToString();
				if (i % 2 > 0)
					dr["Name"] = string.Format("Test Name # {0}", i);
				else
					dr["Name"] = string.Format("aaaaaaaaaaaaaa bbbbbbbbbbbbbTest Name Unmodule # {0}", i);

				dt.Rows.Add(dr);
			}
			return dt;

		}

For the time being the select can be skipped from decoration:
<telerik:RadFormDecorator runat="server" ID="rfd" DecoratedControls="All" ControlsToSkip="Select"/>

There is also a script attached that provides a workaround for this issue.

Declined with the following reason: elements are styled through CSS cascade. In order to handle that you can override explicitly RadFormDecororator's styles. For example:
CSS:
	<style>
		@media screen and (-webkit-min-device-pixel-ratio:0) {
			.RadForm.rfdCheckbox input[type="checkbox"].rlbCheck,
			.RadForm.rfdCheckbox input[type="checkbox"].radPreventDecorate,
			.RadForm.rfdCheckbox input[type="radio"].radPreventDecorate {
				width: 13px;
				height: 13px;
			}

			.RadForm.rfdCheckbox input[type="checkbox"].rlbCheck,
			.RadForm.rfdCheckbox input[type="checkbox"].radPreventDecorate {
				-webkit-appearance: checkbox;
			}

			.RadForm.rfdCheckbox input[type="radio"].radPreventDecorate {
				-webkit-appearance: radio;
			}
		}
	</style>
ASPX:
	<form id="form1" runat="server">
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" Skin="Silk" />
		<input type="radio" name="radio-set" id="Radio1" class="radPreventDecorate" />
		<input type="checkbox" name="checkbox-set" id="Checkbox1" class="radPreventDecorate" />
		<telerik:RadListBox ID="RadListBox1" runat="server" CheckBoxes="true">
			<Items>
				<telerik:RadListBoxItem Text="item 1" />
				<telerik:RadListBoxItem Text="item 2" />
				<telerik:RadListBoxItem Text="item 3" />
			</Items>
		</telerik:RadListBox>
	</form>

Rejected with the following comment:

When trying to enable/disable an input element that is decorated with RadFormDecorator, the disabled attribute must be set to the DOM input element and not to the JavaScript input object, in order for the RadFormDecorator to take effect. For example:
ASPX:
<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
<telerik:RadButton ID="RadButton1" runat="server" AutoPostBack="false" OnClientClicked="disableCheckBox" Text="click" />
<input type="checkbox" id="checkbox" name="name" value="sdf" checked="checked" />
JavaScript:
function disableCheckBox() {
    var checkbox1 = $telerik.$('#checkbox');
    //Either
    //checkbox1.prop('disabled', 'disabled');
    //OR
    checkbox1[0].disabled = true;
}
In IE there is an issue with the disabled property of the DOM elements, when it comes to tracking it. This issue is reproduced only when the element is enabled and should become disabled. The other way around the tracking is working, i.e. if the select is disabled it can be enabled and IE will raise an event about it. This is a browser issue and there is nothing that can be done on our side to work around it.

Improve the appearance of decorated Select and ASP DropDownList by RadFormDecorator in RenderMode="Classic", so that their look resembles RadDropDownList.

For the time being RenderMode="Lightweight" can be used.

Rejected with comment:
Future appearance improvements for RadFormDecorator in Classic mode will not be availalble anymore. The expection will be issues related to functionality or critical appearance issues.