Sounds like you need a StretchPanel
. Silverlight does not have such a panel, but you can write your own.
I found one via google StretchPanel blog, just had a quick look at the code and it looks ok to me.
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StretchPanel Orientation="Horizontal"/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>