BorderImage 100% width and height of parent

In my UI I want to mark a thumb nail, a button, as selected. I do this by adding a BorderImage as a child of the thumb button. How can I make this BorderImage have a width and height of 100% of it’s parent container?

40

My code looks like this

<element type="Button" style="gridButton">
		<attribute name="Name" value="BridgeWood1" />
		<attribute name="Texture" value="Texture2D;Textures/BridgeWood_DIFFUSE.256.jpg" />
		<element type="Text" style="gridButtonText">
				<attribute name="Text" value="Bridge Wood" />
		</element>
		<element type="BorderImage">
				<attribute name="Min Size" value="100 100" />
				<attribute name="Max Size" value="100 100" />
				<attribute name="Border" value="4 4 4 4" />
				<attribute name="Image Rect" value="32 64 48 80" />
				<attribute name="Texture" value="Texture2D;Textures/HiberUI.png" />
		</element>
</element>