Thanks jaga.
I have actually got a Absolute panel in the layer. This has a width and height of 100%. The child of the absolute panel is another absolute panel (restricted here as I am doing drag and drop using the GWT D'N'D library). It is the child absolute panel which is ending up with a height of zero.
I'm not entirely sure but I may have to create an extension of AbsolutePanel that imlpements the ProvidesResize method by the sounds of it.