Width and height properties for the legend that will provide firm dimensions in pixels, so regardless of the series names' length, it will always have constant size.
You can set width and height through the kendo widget:
		<script>
			function pageLoad() {
				var chart = $find("<%=DonutChart.ClientID%>").get_kendoWidget();
				chart.options.legend.height = 80;
				chart.redraw();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="DonutChart" Width="500" Height="500" Transitions="true">
			<Appearance>
				<FillStyle BackgroundColor="White"></FillStyle>
			</Appearance>
			<ChartTitle Text="OS Usage Statistics for December 2012">
				<Appearance Align="Center" Position="Top"></Appearance>
			</ChartTitle>
			<PlotArea>
				<Series>
					<telerik:DonutSeries>
						<LabelsAppearance Visible="false">
						</LabelsAppearance>
						<TooltipsAppearance DataFormatString="{0}%" BackgroundColor="White"></TooltipsAppearance>
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="#00adcc" Name="Win7"
								Y="55.6"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#cccccc" Name="Win8" Y="2.5"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#999999" Name="Vista" Y="2.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#888888" Name="NT" Y="1.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#777777" Name="WinXP" Y="21.1"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#666666" Name="Linux" Y="4.7"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#555555" Name="Mac" Y="8.7"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#444444" Name="Mobile" Y="2.2"></telerik:PieSeriesItem>
						</SeriesItems>
					</telerik:DonutSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>