I am not quite sure from your description if you actually contracted nodes into some kind of super nodes -- which means the original nodes no longer exist -- or whether you used the "group nodes" feature -- so the original nodes still exist, but are contained in special group nodes.
In the first case, you can set the size or any other visual feature of these super nodes just like any other nodes, and especially you can do so based on some attribute in the visualization -> mapping tab. So this means you only need an attribute that tells you how many original nodes were contracted into the super node. To produce such an attribute in visone, navigate to the transformation -> nodes -> merge tab and select the "contract nodes" operation. In that tab, you should also check the "create multiplicity" box at the bottom. Whenever you then apply the contraction operation to your currently selected nodes via this tab, the multiplicity attribute will later tell you for the resulting super node how many nodes were contracted into it. You can then use this multiplicity attribute to set some visual feature in the visualization -> mapping tab. (To select all nodes in the same category quickly, I suggest to use the “select” feature in the attribute manager if you have these categories represented in the form of an attribute. You can access the attribute manager from the toolbar at the top.)
If instead you used the "group nodes" feature and created group nodes, then you can still produce the same kind of attribute when creating group nodes (just select "create group nodes" instead of "contract nodes" as the kind of merge operation in the same tab). However, you cannot affect all visual features of group nodes like the visual features of normal nodes. That is because size, shape and position of these group nodes are fixed by the positioning, shape and size of the individual nodes they are created from -- which means you have to change the position, shape or size of the contained nodes to change position, shape or size of the group node. However, you could still set the color and label of the group nodes according to some attribute value, independent of the color and labels of the contained nodes.
Note that you can also select some standard ways to aggregate the attributes of the individual nodes in the "merge nodes" tab, just in case you have a need for this in your analysis, too.
Hope this helps!