As a workaround the width of the range placeholder can be set through the gaugeObject:
JavaScript:
		function pageLoad() {
			var gauge = $find("<%=RadRadialGauge1.ClientID%>");
			gauge._gaugeObject.options.scale.rangeSize = 30;
			gauge.repaint();
		}
		<telerik:RadRadialGauge runat="server" ID="RadRadialGauge1" Height="350px" Width="350px">
			<Pointer Value="220" Color="Black">
				<Cap Color="Gray" Size="0.1" />
			</Pointer>
			<Scale Min="120" Max="300" MajorUnit="30" MinorUnit="5">
				<Labels Format="{0} °C" Position="Outside" />
				<Ranges>
					<telerik:GaugeRange Color="Orange" From="150" To="190" />
					<telerik:GaugeRange Color="Green" From="190" To="230" />
					<telerik:GaugeRange Color="Red" From="230" To="300" />
				</Ranges>
			</Scale>
		</telerik:RadRadialGauge>