Each node, including the Association node, is documented in Enterprise Miner Help, which is accessible from the main menu: Help > Contents.
Search "Association node" and then scroll to the section called "Link Graph". Also see the Examples section in Help.
In a nutshell, node size and color reflects their counts (larger nodes have higher frequency). Link color and width is tied to strength of association between nodes (thicker = higher confidence level).
With the Link Graph open, you may find it useful to view the data tables used to construct the graph. To do so, click on the link graph, then click on the Table icon in the toolbar in Results view. You will see two tables, one containing node frequencies and the other containing link data. You should be able to see how the data maps to node and link size.
Hope this helps.
