To make an object parametric, you have to use an input node. For example, you can create a Length node and connect it to the Size X input of the Block node.
Now the value can be set by modifying the value of the Length node. To make an object parameter from the Length node, open the settings dialog and set the Create Parameter value to true. You can also set a name for this parameter. This name will be visible in the Object Settings Dialog.
You can use as many parameters as you wish for your objects.
Using Standard Parameters
There are some special parameters in ARCHICAD: Dimension 1, Dimension 2 and Height. Set the value of these parameters in the Preview and Positioning page of the Object Settings Dialog. The real size of a well-designed library object is always synchronized with these values.
To use the value of these parameters, use the corresponding nodes from the Parameters group in the editor. The default value of these nodes can be set below the Node Tree.
Available Nodes
See PARAM-O Node Documentation for details on all node types.