You can allow the Popup Window of the RadPickerBox control to take the entire screen. This behavior is default for the popup, so if you want to disable it, use
the IsFullScreen property and set it to False.
When enabled the FullScreen mode will take precedence over the other properties regarding the popup's
placement. To learn more about them read the Popup Placement and the Popup Size Modes topics.
Here is an example.
<telerikPrimitives:RadPickerBox x:Name="radPickerBox" IsFullScreen="False" />