I would like to create a widget for an extension what uses the <optiongroup> to display page size. 

so something like:

<param name="Page Size" type="optiongroup" appearance="combo"
gui-text="Page Size">
   <option value="1">A4</option>
   <option value="2">A3</option>
    ....
    .....
</param>

<param name="Custom Width" type="float" precision="1" min="0" max="9999" 
gui-text="Custom page width ">200.0</param>

<param name="Custom Height" type="float" precision="1" min="0" max="9999" gui-text="Custom page  height">290.0</param>

What I would like to have is a list of page sizes plus an option for a custom page size, very similar to the current "Document properties" tab within Inkscape. In this tab the value for the page size that is displayed in the "custom page size" input fields is automatically updated when a new page size is selected from the preset menu (A3, A4, ...etc).
I would like to create the same effect for the widget for my extension, that is, update the input fields in the widget each time a new page size preset is selected.

How can I achieve this kind of behaviuor ?