private double getNodeSize(final TreeNode treeNode) {
        return getWidthOrHeightOfNode(treeNode, isLevelChangeInYAxis());
    }
When the level changes in Y-axis (i.e. root location Top or Bottom) the
width of a node is its size, otherwise the node's height is its size.
<p>
The size of a node is used when calculating the distance between two
nodes.

@param treeNode
@return
stroom-core-client-widget/src/main/java/stroom/widget/htree/client/treelayout/AbegoTreeLayout.java