After dragging a default RadDropDownList onto a winForm I added 11 elements. It would only show about 5 then autoscroll. I changed the DropDownMinSize's Height property to 200 and I got the following:
(http://www.sogentech.com/images/11Elements.jpg)
Judging by the link you gave, I expect you are developing for the web. Here is a short explanation about controlling size from Lancelot over on the Telerik site. I have a feeling the MinSize or equivalent setting (maybe MinDropDownHeight) is what you want to play with. The following code is specifically for Silverlight, but this should get you started on the right path:
<!-- Here I've inserted a RadComboBox, I've also set some initial properties-->
<telerik:RadComboBox Margin="211,59,179,0"
VerticalAlignment="Center"
HorizontalAlignment="Center"
MaxDropDownHeight="225"
MinWidth="100"
ScrollViewer.HorizontalScrollBarVisibility="Auto"
ScrollViewer.VerticalScrollBarVisibility="Auto"
IsMouseWheelEnabled="False"
Text="Top Text Title"
EmptyText="empty">
<!-- This is the default style, it uses the parent container's properties such as width and height -->
<telerik:RadComboBoxItem Content="First list Item"/>
<!-- You can explicitly change and set the height/width of the list item by doing this-->
<telerik:RadComboBoxItem Content="Second List Item"
Height="100"
Width="100"/>
<!-- Or you can do what I've done here. I set a Minimum width and height and then set the scrollBar to "Auto"-->
<telerik:RadComboBoxItem Content="Third list item"
ScrollViewer.HorizontalScrollBarVisibility="Auto"
ScrollViewer.VerticalScrollBarVisibility="Auto"
MinHeight="50"
MinWidth="200"/>
</telerik:RadComboBox>
</Grid>